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In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a
file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
, while large databases are hosted on
computer clusters A computer cluster is a set of computers that work together so that they can be viewed as a single system. Unlike grid computers, computer clusters have each node set to perform the same task, controlled and scheduled by software. The comp ...
or
cloud storage Cloud storage is a model of computer data storage in which the digital data is stored in logical pools, said to be on "the cloud". The physical storage spans multiple servers (sometimes in multiple locations), and the physical environment is t ...
. The design of databases spans formal techniques and practical considerations, including
data modeling Data modeling in software engineering is the process of creating a data model for an information system by applying certain formal techniques. Overview Data modeling is a process used to define and analyze data requirements needed to suppo ...
, efficient data representation and storage, query languages,
security Security is protection from, or resilience against, potential harm (or other unwanted coercive change) caused by others, by restraining the freedom of others to act. Beneficiaries (technically referents) of security may be of persons and social ...
and
privacy Privacy (, ) is the ability of an individual or group to seclude themselves or information about themselves, and thereby express themselves selectively. The domain of privacy partially overlaps with security, which can include the concepts of a ...
of sensitive data, and distributed computing issues, including supporting concurrent access and fault tolerance. A
database management system In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases span ...
(DBMS) is the software that interacts with
end user In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrat ...
s, applications, and the database itself to capture and analyze the data. The DBMS software additionally encompasses the core facilities provided to administer the database. The sum total of the database, the DBMS and the associated applications can be referred to as a database system. Often the term "database" is also used loosely to refer to any of the DBMS, the database system or an application associated with the database.
Computer scientists Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (including th ...
may classify database management systems according to the
database model A database model is a type of data model that determines the logical structure of a database. It fundamentally determines in which manner data can be stored, organized and manipulated. The most popular example of a database model is the relational ...
s that they support.
Relational database A relational database is a (most commonly digital) database based on the relational model of data, as proposed by E. F. Codd in 1970. A system used to maintain relational databases is a relational database management system (RDBMS). Many relatio ...
s became dominant in the 1980s. These model data as rows and columns in a series of tables, and the vast majority use SQL for writing and querying data. In the 2000s, non-relational databases became popular, collectively referred to as
NoSQL A NoSQL (originally referring to "non- SQL" or "non-relational") database provides a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relational databases. Such databases have existed ...
, because they use different query languages.


Terminology and overview

Formally, a "database" refers to a set of related data and the way it is organized. Access to this data is usually provided by a "database management system" (DBMS) consisting of an integrated set of computer software that allows users to interact with one or more databases and provides access to all of the data contained in the database (although restrictions may exist that limit access to particular data). The DBMS provides various functions that allow entry, storage and retrieval of large quantities of information and provides ways to manage how that information is organized. Because of the close relationship between them, the term "database" is often used casually to refer to both a database and the DBMS used to manipulate it. Outside the world of professional information technology, the term ''database'' is often used to refer to any collection of related data (such as a spreadsheet or a card index) as size and usage requirements typically necessitate use of a database management system. Existing DBMSs provide various functions that allow management of a database and its data which can be classified into four main functional groups: * Data definition – Creation, modification and removal of definitions that define the organization of the data. * Update – Insertion, modification, and deletion of the actual data. * Retrieval – Providing information in a form directly usable or for further processing by other applications. The retrieved data may be made available in a form basically the same as it is stored in the database or in a new form obtained by altering or combining existing data from the database. * Administration – Registering and monitoring users, enforcing data security, monitoring performance, maintaining data integrity, dealing with concurrency control, and recovering information that has been corrupted by some event such as an unexpected system failure. Both a database and its DBMS conform to the principles of a particular
database model A database model is a type of data model that determines the logical structure of a database. It fundamentally determines in which manner data can be stored, organized and manipulated. The most popular example of a database model is the relational ...
. "Database system" refers collectively to the database model, database management system, and database. Physically, database servers are dedicated computers that hold the actual databases and run only the DBMS and related software. Database servers are usually multiprocessor computers, with generous memory and RAID disk arrays used for stable storage. Hardware database accelerators, connected to one or more servers via a high-speed channel, are also used in large volume transaction processing environments. DBMSs are found at the heart of most database applications. DBMSs may be built around a custom multitasking kernel with built-in
networking Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...
support, but modern DBMSs typically rely on a standard operating system to provide these functions. Since DBMSs comprise a significant market, computer and storage vendors often take into account DBMS requirements in their own development plans. Databases and DBMSs can be categorized according to the database model(s) that they support (such as relational or XML), the type(s) of computer they run on (from a
server cluster A computer cluster is a set of computers that work together so that they can be viewed as a single system. Unlike grid computers, computer clusters have each node set to perform the same task, controlled and scheduled by software. The comp ...
to a mobile phone), the query language(s) used to access the database (such as SQL or XQuery), and their internal engineering, which affects performance, scalability, resilience, and security.


History

The sizes, capabilities, and performance of databases and their respective DBMSs have grown in orders of magnitude. These performance increases were enabled by the technology progress in the areas of processors, computer memory,
computer storage Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a compute ...
, and computer networks. The concept of a database was made possible by the emergence of direct access storage media such as magnetic disks, which became widely available in the mid-1960s; earlier systems relied on sequential storage of data on magnetic tape. The subsequent development of database technology can be divided into three eras based on data model or structure:
navigational Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
, SQL/ relational, and post-relational. The two main early navigational data models were the
hierarchical model A hierarchical database model is a data model in which the data are organized into a tree-like structure. The data are stored as records which are connected to one another through links. A record is a collection of fields, with each field containin ...
and the CODASYL model ( network model). These were characterized by the use of pointers (often physical disk addresses) to follow relationships from one record to another. The
relational model The relational model (RM) is an approach to managing data using a Structure (mathematical logic), structure and language consistent with first-order logic, first-order predicate logic, first described in 1969 by English computer scientist Edgar F. ...
, first proposed in 1970 by
Edgar F. Codd Edgar Frank "Ted" Codd (19 August 1923 – 18 April 2003) was an English computer scientist who, while working for IBM, invented the relational model for database management, the theoretical basis for relational databases and relational databa ...
, departed from this tradition by insisting that applications should search for data by content, rather than by following links. The relational model employs sets of ledger-style tables, each used for a different type of entity. Only in the mid-1980s did computing hardware become powerful enough to allow the wide deployment of relational systems (DBMSs plus applications). By the early 1990s, however, relational systems dominated in all large-scale
data processing Data processing is the collection and manipulation of digital data to produce meaningful information. Data processing is a form of ''information processing'', which is the modification (processing) of information in any manner detectable by an ...
applications, and they remain dominant:
IBM Db2 Db2 is a family of data management products, including database servers, developed by IBM. It initially supported the relational model, but was extended to support object–relational features and non-relational structures like JSON a ...
,
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The word '' ...
, MySQL, and Microsoft SQL Server are the most searched
DBMS In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases spa ...
. The dominant database language, standardized SQL for the relational model, has influenced database languages for other data models. Object databases were developed in the 1980s to overcome the inconvenience of
object–relational impedance mismatch The object–relational impedance mismatch is a set of conceptual and technical difficulties that are often encountered when a relational database management system (RDBMS) is being served by an application program (or multiple application progra ...
, which led to the coining of the term "post-relational" and also the development of hybrid
object–relational database An object–relational database (ORD), or object–relational database management system (ORDBMS), is a database management system (DBMS) similar to a relational database, but with an object-oriented database model: objects, classes and inheritan ...
s. The next generation of post-relational databases in the late 2000s became known as
NoSQL A NoSQL (originally referring to "non- SQL" or "non-relational") database provides a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relational databases. Such databases have existed ...
databases, introducing fast key–value stores and document-oriented databases. A competing "next generation" known as NewSQL databases attempted new implementations that retained the relational/SQL model while aiming to match the high performance of NoSQL compared to commercially available relational DBMSs.


1960s, navigational DBMS

The introduction of the term ''database'' coincided with the availability of direct-access storage (disks and drums) from the mid-1960s onwards. The term represented a contrast with the tape-based systems of the past, allowing shared interactive use rather than daily batch processing. The Oxford English Dictionary cites a 1962 report by the System Development Corporation of California as the first to use the term "data-base" in a specific technical sense. As computers grew in speed and capability, a number of general-purpose database systems emerged; by the mid-1960s a number of such systems had come into commercial use. Interest in a standard began to grow, and Charles Bachman, author of one such product, the Integrated Data Store (IDS), founded the Database Task Group within CODASYL, the group responsible for the creation and standardization of
COBOL COBOL (; an acronym for "common business-oriented language") is a compiled English-like computer programming language designed for business use. It is an imperative, procedural and, since 2002, object-oriented language. COBOL is primarily us ...
. In 1971, the Database Task Group delivered their standard, which generally became known as the ''CODASYL approach'', and soon a number of commercial products based on this approach entered the market. The CODASYL approach offered applications the ability to navigate around a linked data set which was formed into a large network. Applications could find records by one of three methods: #Use of a primary key (known as a CALC key, typically implemented by hashing) #Navigating relationships (called ''sets'') from one record to another #Scanning all the records in a sequential order Later systems added B-trees to provide alternate access paths. Many CODASYL databases also added a declarative query language for end users (as distinct from the navigational API). However, CODASYL databases were complex and required significant training and effort to produce useful applications. IBM also had its own DBMS in 1966, known as Information Management System (IMS). IMS was a development of software written for the Apollo program on the
System/360 The IBM System/360 (S/360) is a family of mainframe computer systems that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applica ...
. IMS was generally similar in concept to CODASYL, but used a strict hierarchy for its model of data navigation instead of CODASYL's network model. Both concepts later became known as navigational databases due to the way data was accessed: the term was popularized by Bachman's 1973 Turing Award presentation ''The Programmer as Navigator''. IMS is classified by IBM as a hierarchical database. IDMS and
Cincom Systems Cincom Systems, Inc., is a Private company, privately held multinational computer technology corporation founded in 1968 by Tom Nies, Tom Richley, and Claude Bogardus. The company's best known product today is named Total (trademark ''TOTAL''). ...
'
TOTAL Total may refer to: Mathematics * Total, the summation of a set of numbers * Total order, a partial order without incomparable pairs * Total relation, which may also mean ** connected relation (a binary relation in which any two elements are comp ...
databases are classified as network databases. IMS remains in use .


1970s, relational DBMS

Edgar F. Codd Edgar Frank "Ted" Codd (19 August 1923 – 18 April 2003) was an English computer scientist who, while working for IBM, invented the relational model for database management, the theoretical basis for relational databases and relational databa ...
worked at IBM in San Jose, California, in one of their offshoot offices that were primarily involved in the development of
hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
systems. He was unhappy with the navigational model of the CODASYL approach, notably the lack of a "search" facility. In 1970, he wrote a number of papers that outlined a new approach to database construction that eventually culminated in the groundbreaking ''A Relational Model of Data for Large Shared Data Banks''. In this paper, he described a new system for storing and working with large databases. Instead of records being stored in some sort of
linked list In computer science, a linked list is a linear collection of data elements whose order is not given by their physical placement in memory. Instead, each element points to the next. It is a data structure consisting of a collection of nodes whic ...
of free-form records as in CODASYL, Codd's idea was to organize the data as a number of " tables", each table being used for a different type of entity. Each table would contain a fixed number of columns containing the attributes of the entity. One or more columns of each table were designated as a
primary key In the relational model of databases, a primary key is a ''specific choice'' of a ''minimal'' set of attributes (Column (database), columns) that uniquely specify a tuple (Row (database), row) in a Relation (database), relation (Table (database), t ...
by which the rows of the table could be uniquely identified; cross-references between tables always used these primary keys, rather than disk addresses, and queries would join tables based on these key relationships, using a set of operations based on the mathematical system of
relational calculus The relational calculus consists of two calculi, the tuple relational calculus and the domain relational calculus, that are part of the relational model for databases and provide a declarative way to specify database queries. The raison d'être ...
(from which the model takes its name). Splitting the data into a set of normalized tables (or ''relations'') aimed to ensure that each "fact" was only stored once, thus simplifying update operations. Virtual tables called ''views'' could present the data in different ways for different users, but views could not be directly updated. Codd used mathematical terms to define the model: relations, tuples, and domains rather than tables, rows, and columns. The terminology that is now familiar came from early implementations. Codd would later criticize the tendency for practical implementations to depart from the mathematical foundations on which the model was based. The use of primary keys (user-oriented identifiers) to represent cross-table relationships, rather than disk addresses, had two primary motivations. From an engineering perspective, it enabled tables to be relocated and resized without expensive database reorganization. But Codd was more interested in the difference in semantics: the use of explicit identifiers made it easier to define update operations with clean mathematical definitions, and it also enabled query operations to be defined in terms of the established discipline of first-order predicate calculus; because these operations have clean mathematical properties, it becomes possible to rewrite queries in provably correct ways, which is the basis of query optimization. There is no loss of expressiveness compared with the hierarchic or network models, though the connections between tables are no longer so explicit. In the hierarchic and network models, records were allowed to have a complex internal structure. For example, the salary history of an employee might be represented as a "repeating group" within the employee record. In the relational model, the process of normalization led to such internal structures being replaced by data held in multiple tables, connected only by logical keys. For instance, a common use of a database system is to track information about users, their name, login information, various addresses and phone numbers. In the navigational approach, all of this data would be placed in a single variable-length record. In the relational approach, the data would be ''normalized'' into a user table, an address table and a phone number table (for instance). Records would be created in these optional tables only if the address or phone numbers were actually provided. As well as identifying rows/records using logical identifiers rather than disk addresses, Codd changed the way in which applications assembled data from multiple records. Rather than requiring applications to gather data one record at a time by navigating the links, they would use a declarative query language that expressed what data was required, rather than the access path by which it should be found. Finding an efficient access path to the data became the responsibility of the database management system, rather than the application programmer. This process, called query optimization, depended on the fact that queries were expressed in terms of mathematical logic. Codd's paper was picked up by two people at Berkeley, Eugene Wong and Michael Stonebraker. They started a project known as
INGRES Jean-Auguste-Dominique Ingres ( , ; 29 August 1780 – 14 January 1867) was a French Neoclassicism, Neoclassical Painting, painter. Ingres was profoundly influenced by past artistic traditions and aspired to become the guardian of academic ...
using funding that had already been allocated for a geographical database project and student programmers to produce code. Beginning in 1973, INGRES delivered its first test products which were generally ready for widespread use in 1979. INGRES was similar to System R in a number of ways, including the use of a "language" for data access, known as QUEL. Over time, INGRES moved to the emerging SQL standard. IBM itself did one test implementation of the relational model, PRTV, and a production one,
Business System 12 Business System 12, or simply BS12, was one of the first fully relational database management systems, designed and implemented by IBM's ''Bureau Service'' subsidiary at the company's international development centre in Uithoorn, Netherlands. ...
, both now discontinued. Honeywell wrote MRDS for Multics, and now there are two new implementations: Alphora Dataphor and Rel. Most other DBMS implementations usually called ''relational'' are actually SQL DBMSs. In 1970, the University of Michigan began development of the MICRO Information Management System based on D.L. Childs' Set-Theoretic Data model. MICRO was used to manage very large data sets by the US Department of Labor, the U.S. Environmental Protection Agency, and researchers from the University of Alberta, the University of Michigan, and Wayne State University. It ran on IBM mainframe computers using the Michigan Terminal System. The system remained in production until 1998.


Integrated approach

In the 1970s and 1980s, attempts were made to build database systems with integrated hardware and software. The underlying philosophy was that such integration would provide higher performance at a lower cost. Examples were IBM System/38, the early offering of Teradata, and the
Britton Lee, Inc. Britton Lee Inc. was a pioneering relational database company. Renamed ShareBase, it was acquired by Teradata in June, 1990. History Britton Lee was founded in 1979 by David L. Britton, Geoffrey M. Lee and a group of hardware engineers along with ...
database machine. Another approach to hardware support for database management was
ICL ICL may refer to: Companies and organizations * Idaho Conservation League * Imperial College London, a UK university * Indian Confederation of Labour * Indian Cricket League * Inorganic Chemistry Laboratory of the University of Oxford * Israel Ch ...
's CAFS accelerator, a hardware disk controller with programmable search capabilities. In the long term, these efforts were generally unsuccessful because specialized database machines could not keep pace with the rapid development and progress of general-purpose computers. Thus most database systems nowadays are software systems running on general-purpose hardware, using general-purpose computer data storage. However, this idea is still pursued in certain applications by some companies like
Netezza IBM Netezza (pronounced ne-teez-a) is a subsidiary of American technology company IBM that designs and markets high-performance data warehouse appliances and advanced analytics applications for uses including enterprise data warehousing, busines ...
and Oracle (
Exadata The Oracle Exadata Database Machine (Exadata) is a computing platform optimized for running Oracle Databases. Exadata is a combined hardware and software platform that includes scale-out Intel x86-64 compute and storage servers, RoCE or Infini ...
).


Late 1970s, SQL DBMS

IBM started working on a prototype system loosely based on Codd's concepts as '' System R'' in the early 1970s. The first version was ready in 1974/5, and work then started on multi-table systems in which the data could be split so that all of the data for a record (some of which is optional) did not have to be stored in a single large "chunk". Subsequent multi-user versions were tested by customers in 1978 and 1979, by which time a standardized query language – SQL – had been added. Codd's ideas were establishing themselves as both workable and superior to CODASYL, pushing IBM to develop a true production version of System R, known as ''SQL/DS'', and, later, ''Database 2'' (
IBM Db2 Db2 is a family of data management products, including database servers, developed by IBM. It initially supported the relational model, but was extended to support object–relational features and non-relational structures like JSON a ...
). Larry Ellison's Oracle Database (or more simply,
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The word '' ...
) started from a different chain, based on IBM's papers on System R. Though Oracle V1 implementations were completed in 1978, it wasn't until Oracle Version 2 when Ellison beat IBM to market in 1979. Stonebraker went on to apply the lessons from INGRES to develop a new database, Postgres, which is now known as
PostgreSQL PostgreSQL (, ), also known as Postgres, is a free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance. It was originally named POSTGRES, referring to its origins as a successor to the In ...
. PostgreSQL is often used for global mission-critical applications (the .org and .info domain name registries use it as their primary data store, as do many large companies and financial institutions). In Sweden, Codd's paper was also read and
Mimer SQL Mimer SQL is an SQL-based relational database management system produced by the Swedish company ''Mimer Information Technology AB'' (Mimer AB), formerly known as ''Upright Database Technology AB''. It was originally developed as a research proje ...
was developed in the mid-1970s at Uppsala University. In 1984, this project was consolidated into an independent enterprise. Another data model, the entity–relationship model, emerged in 1976 and gained popularity for
database design Database design is the organization of data according to a database model. The designer determines what data must be stored and how the data elements interrelate. With this information, they can begin to fit the data to the database model.Teorey, ...
as it emphasized a more familiar description than the earlier relational model. Later on, entity–relationship constructs were retrofitted as a
data modeling Data modeling in software engineering is the process of creating a data model for an information system by applying certain formal techniques. Overview Data modeling is a process used to define and analyze data requirements needed to suppo ...
construct for the relational model, and the difference between the two has become irrelevant.


1980s, on the desktop

The 1980s ushered in the age of desktop computing. The new computers empowered their users with spreadsheets like Lotus 1-2-3 and database software like
dBASE dBase (also stylized dBASE) was one of the first database management systems for microcomputers and the most successful in its day. The dBase system includes the core database engine, a query system, a forms engine, and a programming language ...
. The dBASE product was lightweight and easy for any computer user to understand out of the box.
C. Wayne Ratliff Cecil Wayne Ratliff (born 1946) wrote the database program Vulcan. Raised in Germany and the US, he now resides in the Los Angeles area. Biography Ratliff was born in 1946 in Trenton, Ohio, USA. From 1969 to 1982 he worked for the Martin Mariett ...
, the creator of dBASE, stated: "dBASE was different from programs like BASIC, C, FORTRAN, and COBOL in that a lot of the dirty work had already been done. The data manipulation is done by dBASE instead of by the user, so the user can concentrate on what he is doing, rather than having to mess with the dirty details of opening, reading, and closing files, and managing space allocation." dBASE was one of the top selling software titles in the 1980s and early 1990s.


1990s, object-oriented

The 1990s, along with a rise in object-oriented programming, saw a growth in how data in various databases were handled. Programmers and designers began to treat the data in their databases as
objects Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...
. That is to say that if a person's data were in a database, that person's attributes, such as their address, phone number, and age, were now considered to belong to that person instead of being extraneous data. This allows for relations between data to be related to objects and their attributes and not to individual fields. The term "
object–relational impedance mismatch The object–relational impedance mismatch is a set of conceptual and technical difficulties that are often encountered when a relational database management system (RDBMS) is being served by an application program (or multiple application progra ...
" described the inconvenience of translating between programmed objects and database tables. Object databases and
object–relational database An object–relational database (ORD), or object–relational database management system (ORDBMS), is a database management system (DBMS) similar to a relational database, but with an object-oriented database model: objects, classes and inheritan ...
s attempt to solve this problem by providing an object-oriented language (sometimes as extensions to SQL) that programmers can use as alternative to purely relational SQL. On the programming side, libraries known as
object–relational mapping Object–relational mapping (ORM, O/RM, and O/R mapping tool) in computer science is a programming technique for converting data between type systems using object-oriented programming languages. This creates, in effect, a "virtual object databa ...
s (ORMs) attempt to solve the same problem.


2000s, NoSQL and NewSQL

XML databases are a type of structured document-oriented database that allows querying based on XML document attributes. XML databases are mostly used in applications where the data is conveniently viewed as a collection of documents, with a structure that can vary from the very flexible to the highly rigid: examples include scientific articles, patents, tax filings, and personnel records.
NoSQL A NoSQL (originally referring to "non- SQL" or "non-relational") database provides a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relational databases. Such databases have existed ...
databases are often very fast, do not require fixed table schemas, avoid join operations by storing denormalized data, and are designed to scale horizontally. In recent years, there has been a strong demand for massively distributed databases with high partition tolerance, but according to the
CAP theorem In theoretical computer science, the CAP theorem, also named Brewer's theorem after computer scientist Eric Brewer, states that any distributed data store can provide only two of the following three guarantees:Seth Gilbert and Nancy Lynch"Brewer' ...
, it is impossible for a distributed system to simultaneously provide consistency, availability, and partition tolerance guarantees. A distributed system can satisfy any two of these guarantees at the same time, but not all three. For that reason, many NoSQL databases are using what is called eventual consistency to provide both availability and partition tolerance guarantees with a reduced level of data consistency. NewSQL is a class of modern relational databases that aims to provide the same scalable performance of NoSQL systems for online transaction processing (read-write) workloads while still using SQL and maintaining the
ACID In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
guarantees of a traditional database system.


Use cases

Databases are used to support internal operations of organizations and to underpin online interactions with customers and suppliers (see Enterprise software). Databases are used to hold administrative information and more specialized data, such as engineering data or economic models. Examples include computerized library systems, flight reservation systems, computerized
parts inventory system Inventory control or stock control can be broadly defined as "the activity of checking a shop's stock". It is the process of ensuring that the right amount of supply is available within a business. However, a more focused definition takes into acco ...
s, and many
content management system A content management system (CMS) is computer software used to manage the creation and modification of digital content (content management).''Managing Enterprise Content: A Unified Content Strategy''. Ann Rockley, Pamela Kostur, Steve Manning. New ...
s that store websites as collections of webpages in a database.


Classification

One way to classify databases involves the type of their contents, for example: bibliographic, document-text, statistical, or multimedia objects. Another way is by their application area, for example: accounting, music compositions, movies, banking, manufacturing, or insurance. A third way is by some technical aspect, such as the database structure or interface type. This section lists a few of the adjectives used to characterize different kinds of databases. * An in-memory database is a database that primarily resides in
main memory Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a computer ...
, but is typically backed-up by non-volatile computer data storage. Main memory databases are faster than disk databases, and so are often used where response time is critical, such as in telecommunications network equipment. * An active database includes an event-driven architecture which can respond to conditions both inside and outside the database. Possible uses include security monitoring, alerting, statistics gathering and authorization. Many databases provide active database features in the form of database triggers. * A cloud database relies on
cloud technology Cloud computing is the on-demand availability of computer system resources, especially data storage ( cloud storage) and computing power, without direct active management by the user. Large clouds often have functions distributed over mul ...
. Both the database and most of its DBMS reside remotely, "in the cloud", while its applications are both developed by programmers and later maintained and used by end-users through a web browser and Open APIs. * Data warehouses archive data from operational databases and often from external sources such as market research firms. The warehouse becomes the central source of data for use by managers and other end-users who may not have access to operational data. For example, sales data might be aggregated to weekly totals and converted from internal product codes to use UPCs so that they can be compared with
ACNielsen The Nielsen Corporation, self-referentially known as The Nielsen Company, and formerly known as ACNielsen or AC Nielsen, is a global marketing research firm, with worldwide headquarters in New York City, United States. Regional headquarters for ...
data. Some basic and essential components of data warehousing include extracting, analyzing, and mining data, transforming, loading, and managing data so as to make them available for further use. * A deductive database combines logic programming with a relational database. * A distributed database is one in which both the data and the DBMS span multiple computers. * A document-oriented database is designed for storing, retrieving, and managing document-oriented, or semi structured, information. Document-oriented databases are one of the main categories of NoSQL databases. * An embedded database system is a DBMS which is tightly integrated with an application software that requires access to stored data in such a way that the DBMS is hidden from the application's end-users and requires little or no ongoing maintenance. *End-user databases consist of data developed by individual end-users. Examples of these are collections of documents, spreadsheets, presentations, multimedia, and other files. Several products exist to support such databases. Some of them are much simpler than full-fledged DBMSs, with more elementary DBMS functionality. * A federated database system comprises several distinct databases, each with its own DBMS. It is handled as a single database by a federated database management system (FDBMS), which transparently integrates multiple autonomous DBMSs, possibly of different types (in which case it would also be a
heterogeneous database system A heterogeneous database system is an automated (or semi-automated) system for the integration of heterogeneous, disparate database management systems to present a user with a single, unified query interface. Heterogeneous database systems (HDBs) ...
), and provides them with an integrated conceptual view. * Sometimes the term ''multi-database'' is used as a synonym for federated database, though it may refer to a less integrated (e.g., without an FDBMS and a managed integrated schema) group of databases that cooperate in a single application. In this case, typically middleware is used for distribution, which typically includes an atomic commit protocol (ACP), e.g., the two-phase commit protocol, to allow distributed (global) transactions across the participating databases. * A graph database is a kind of NoSQL database that uses graph structures with nodes, edges, and properties to represent and store information. General graph databases that can store any graph are distinct from specialized graph databases such as
triplestore A triplestore or RDF store is a purpose-built database for the storage and retrieval of triples through semantic queries. A triple is a data entity composed of subject–predicate–object, like "Bob is 35" or "Bob knows Fred". Much like a relati ...
s and network databases. * An
array DBMS Array database management systems (array DBMSs) provide database services specifically for arrays (also called raster data), that is: homogeneous collections of data items (often called pixels, voxels, etc.), sitting on a regular grid of one, two, ...
is a kind of NoSQL DBMS that allows modeling, storage, and retrieval of (usually large) multi-dimensional arrays such as satellite images and climate simulation output. * In a
hypertext Hypertext is E-text, text displayed on a computer display or other electronic devices with references (hyperlinks) to other text that the reader can immediately access. Hypertext documents are interconnected by hyperlinks, which are typi ...
or
hypermedia Hypermedia, an extension of the term hypertext, is a nonlinear medium of information that includes graphics, audio, video, plain text and hyperlinks. This designation contrasts with the broader term ''multimedia'', which may include non-interac ...
database, any word or a piece of text representing an object, e.g., another piece of text, an article, a picture, or a film, can be
hyperlink In computing, a hyperlink, or simply a link, is a digital reference to data that the user can follow or be guided by clicking or tapping. A hyperlink points to a whole document or to a specific element within a document. Hypertext is text wit ...
ed to that object. Hypertext databases are particularly useful for organizing large amounts of disparate information. For example, they are useful for organizing online encyclopedias, where users can conveniently jump around the text. The World Wide Web is thus a large distributed hypertext database. * A knowledge base (abbreviated KB, kb or Δ) is a special kind of database for knowledge management, providing the means for the computerized collection, organization, and
retrieval Retrieval could refer to: Computer science * RETRIEVE, Tymshare database that inspired dBASE and others * Data retrieval * Document retrieval * Image retrieval * Information retrieval * Knowledge retrieval * Medical retrieval * Music informati ...
of knowledge. Also a collection of data representing problems with their solutions and related experiences. * A mobile database can be carried on or synchronized from a mobile computing device. * Operational databases store detailed data about the operations of an organization. They typically process relatively high volumes of updates using transactions. Examples include customer databases that record contact, credit, and demographic information about a business's customers, personnel databases that hold information such as salary, benefits, skills data about employees, enterprise resource planning systems that record details about product components, parts inventory, and financial databases that keep track of the organization's money, accounting and financial dealings. * A
parallel database A parallel database system seeks to improve performance through parallelization of various operations, such as loading data, building indexes and evaluating queries. Although data may be stored in a distributed fashion, the distribution is governed ...
seeks to improve performance through parallelization for tasks such as loading data, building indexes and evaluating queries. ::The major parallel DBMS architectures which are induced by the underlying hardware architecture are: ::* Shared memory architecture, where multiple processors share the main memory space, as well as other data storage. ::* Shared disk architecture, where each processing unit (typically consisting of multiple processors) has its own main memory, but all units share the other storage. ::* Shared-nothing architecture, where each processing unit has its own main memory and other storage. *
Probabilistic database Most real databases contain data whose correctness is uncertain. In order to work with such data, there is a need to quantify the integrity of the data. This is achieved by using probabilistic databases. A probabilistic database is an uncertain da ...
s employ
fuzzy logic Fuzzy logic is a form of many-valued logic in which the truth value of variables may be any real number between 0 and 1. It is employed to handle the concept of partial truth, where the truth value may range between completely true and completely ...
to draw inferences from imprecise data. *
Real-time database A real-time database is a database system which uses Real-time computing, real-time processing to handle workloads whose state is constantly changing. This differs from traditional databases containing Persistence (computer science), persistent data ...
s process transactions fast enough for the result to come back and be acted on right away. * A spatial database can store the data with multidimensional features. The queries on such data include location-based queries, like "Where is the closest hotel in my area?". * A temporal database has built-in time aspects, for example a temporal data model and a temporal version of SQL. More specifically the temporal aspects usually include valid-time and transaction-time. * A
terminology-oriented database A terminology-oriented database or terminology-oriented database management system is a conceptual extension of an object-oriented database. It implements concepts defined in a terminology model. Compared with object-oriented databases, the termino ...
builds upon an
object-oriented database An object database or object-oriented database is a database management system in which information is represented in the form of objects as used in object-oriented programming. Object databases are different from relational databases which are ...
, often customized for a specific field. * An unstructured data database is intended to store in a manageable and protected way diverse objects that do not fit naturally and conveniently in common databases. It may include email messages, documents, journals, multimedia objects, etc. The name may be misleading since some objects can be highly structured. However, the entire possible object collection does not fit into a predefined structured framework. Most established DBMSs now support unstructured data in various ways, and new dedicated DBMSs are emerging.


Database management system

Connolly and Begg define database management system (DBMS) as a "software system that enables users to define, create, maintain and control access to the database". Examples of DBMS's include MySQL, MariaDB,
PostgreSQL PostgreSQL (, ), also known as Postgres, is a free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance. It was originally named POSTGRES, referring to its origins as a successor to the In ...
, Microsoft SQL Server,
Oracle Database Oracle Database (commonly referred to as Oracle DBMS, Oracle Autonomous Database, or simply as Oracle) is a multi-model database management system produced and marketed by Oracle Corporation. It is a database commonly used for running online t ...
, and Microsoft Access. The DBMS acronym is sometimes extended to indicate the underlying
database model A database model is a type of data model that determines the logical structure of a database. It fundamentally determines in which manner data can be stored, organized and manipulated. The most popular example of a database model is the relational ...
, with RDBMS for the relational, OODBMS for the object (oriented) and ORDBMS for the object–relational model. Other extensions can indicate some other characteristics, such as DDBMS for a distributed database management systems. The functionality provided by a DBMS can vary enormously. The core functionality is the storage, retrieval and update of data. Codd proposed the following functions and services a fully-fledged general purpose DBMS should provide: * Data storage, retrieval and update * User accessible catalog or data dictionary describing the metadata * Support for transactions and concurrency * Facilities for recovering the database should it become damaged * Support for authorization of access and update of data * Access support from remote locations * Enforcing constraints to ensure data in the database abides by certain rules It is also generally to be expected the DBMS will provide a set of utilities for such purposes as may be necessary to administer the database effectively, including import, export, monitoring, defragmentation and analysis utilities. The core part of the DBMS interacting between the database and the application interface sometimes referred to as the database engine. Often DBMSs will have configuration parameters that can be statically and dynamically tuned, for example the maximum amount of main memory on a server the database can use. The trend is to minimize the amount of manual configuration, and for cases such as embedded databases the need to target zero-administration is paramount. The large major enterprise DBMSs have tended to increase in size and functionality and have involved up to thousands of human years of development effort throughout their lifetime. Early multi-user DBMS typically only allowed for the application to reside on the same computer with access via terminals or terminal emulation software. The client–server architecture was a development where the application resided on a client desktop and the database on a server allowing the processing to be distributed. This evolved into a multitier architecture incorporating
application server An application server is a server that hosts applications or software that delivers a business application through a communication protocol. An application server framework is a service layer model. It includes software components available to a ...
s and
web server A web server is computer software and underlying hardware that accepts requests via HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initiate ...
s with the end user interface via a web browser with the database only directly connected to the adjacent tier. A general-purpose DBMS will provide public
application programming interface An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how t ...
s (API) and optionally a processor for
database language In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases span ...
s such as SQL to allow applications to be written to interact with and manipulate the database. A special purpose DBMS may use a private API and be specifically customized and linked to a single application. For example, an email system performs many of the functions of a general-purpose DBMS such as message insertion, message deletion, attachment handling, blocklist lookup, associating messages an email address and so forth however these functions are limited to what is required to handle email.


Application

External interaction with the database will be via an application program that interfaces with the DBMS. This can range from a database tool that allows users to execute SQL queries textually or graphically, to a website that happens to use a database to store and search information.


Application program interface

A
programmer A computer programmer, sometimes referred to as a software developer, a software engineer, a programmer or a coder, is a person who creates computer programs — often for larger computer software. A programmer is someone who writes/creates ...
will
code In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communication ...
interactions to the database (sometimes referred to as a
datasource DataSource is a name given to the connection set up to a database from a server. The name is commonly used when creating a query to the database. The data source name (DSN) need not be the same as the filename for the database. For example, a da ...
) via an
application program interface An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how t ...
(API) or via a
database language In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases span ...
. The particular API or language chosen will need to be supported by DBMS, possibly indirectly via a preprocessor or a bridging API. Some API's aim to be database independent,
ODBC In computing, Open Database Connectivity (ODBC) is a standard application programming interface (API) for accessing database management systems (DBMS). The designers of ODBC aimed to make it independent of database systems and operating systems. An ...
being a commonly known example. Other common API's include JDBC and ADO.NET.


Database languages

Database languages are special-purpose languages, which allow one or more of the following tasks, sometimes distinguished as
sublanguage A sublanguage is a subset of a language. Sublanguages occur in natural language, computer programming language, and relational databases. In natural language In informatics, natural language processing, and machine translation, a sublanguage is ...
s: * Data control language (DCL) – controls access to data; * Data definition language (DDL) – defines data types such as creating, altering, or dropping tables and the relationships among them; * Data manipulation language (DML) – performs tasks such as inserting, updating, or deleting data occurrences; *
Data query language Data Query Language (DQL) is part of the base grouping of SQL sub-languages. These sub-languages are mainly categorized into four categories: a data query language (DQL), a data definition language (DDL), a data control language (DCL), and a data ...
(DQL) – allows searching for information and computing derived information. Database languages are specific to a particular data model. Notable examples include: * SQL combines the roles of data definition, data manipulation, and query in a single language. It was one of the first commercial languages for the relational model, although it departs in some respects from the relational model as described by Codd (for example, the rows and columns of a table can be ordered). SQL became a standard of the
American National Standards Institute The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
(ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987. The standards have been regularly enhanced since and are supported (with varying degrees of conformance) by all mainstream commercial relational DBMSs. *
OQL Object Query Language (OQL) is a query language standard for object-oriented databases modeled after SQL and developed by the Object Data Management Group (ODMG). Because of its overall complexity the complete OQL standard has not yet been fully ...
is an object model language standard (from the Object Data Management Group). It has influenced the design of some of the newer query languages like JDOQL and
EJB QL EJB QL or EJB-QL is a portable database query language for Enterprise Java Beans. It was used in Java EE applications. Compared to SQL, however, it is less complex but less powerful as well. History The language has been inspired, especially EJB3 ...
. * XQuery is a standard XML query language implemented by XML database systems such as
MarkLogic MarkLogic Corporation is an American software business that develops and provides an enterprise NoSQL database, also named ''MarkLogic''. The company was founded in 2001 and is based in San Carlos, California. MarkLogic is a privately held compa ...
and
eXist eXist-db (or eXist for short) is an open source software project for NoSQL databases built on XML technology. It is classified as both a NoSQL document-oriented database system and a native XML database (and it provides support for XML, JSON, HTM ...
, by relational databases with XML capability such as Oracle and Db2, and also by in-memory XML processors such as
Saxon The Saxons ( la, Saxones, german: Sachsen, ang, Seaxan, osx, Sahson, nds, Sassen, nl, Saksen) were a group of Germanic * * * * peoples whose name was given in the early Middle Ages to a large country (Old Saxony, la, Saxonia) near the Nor ...
. * SQL/XML combines XQuery with SQL. A database language may also incorporate features like: * DBMS-specific configuration and storage engine management * Computations to modify query results, like counting, summing, averaging, sorting, grouping, and cross-referencing * Constraint enforcement (e.g. in an automotive database, only allowing one engine type per car) * Application programming interface version of the query language, for programmer convenience


Storage

Database storage is the container of the physical materialization of a database. It comprises the ''internal'' (physical) ''level'' in the database architecture. It also contains all the information needed (e.g.,
metadata Metadata is "data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive metadata – the descriptive ...
, "data about the data", and internal
data structure In computer science, a data structure is a data organization, management, and storage format that is usually chosen for efficient access to data. More precisely, a data structure is a collection of data values, the relationships among them, a ...
s) to reconstruct the ''conceptual level'' and ''external level'' from the internal level when needed. Databases as digital objects contain three layers of information which must be stored: the data, the structure, and the semantics. Proper storage of all three layers is needed for future
preservation Preservation may refer to: Heritage and conservation * Preservation (library and archival science), activities aimed at prolonging the life of a record while making as few changes as possible * ''Preservation'' (magazine), published by the Nat ...
and longevity of the database. Putting data into permanent storage is generally the responsibility of the database engine a.k.a. "storage engine". Though typically accessed by a DBMS through the underlying operating system (and often using the operating systems'
file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
s as intermediates for storage layout), storage properties and configuration settings are extremely important for the efficient operation of the DBMS, and thus are closely maintained by database administrators. A DBMS, while in operation, always has its database residing in several types of storage (e.g., memory and external storage). The database data and the additional needed information, possibly in very large amounts, are coded into bits. Data typically reside in the storage in structures that look completely different from the way the data look at the conceptual and external levels, but in ways that attempt to optimize (the best possible) these levels' reconstruction when needed by users and programs, as well as for computing additional types of needed information from the data (e.g., when querying the database). Some DBMSs support specifying which character encoding was used to store data, so multiple encodings can be used in the same database. Various low-level database storage structures are used by the storage engine to serialize the data model so it can be written to the medium of choice. Techniques such as indexing may be used to improve performance. Conventional storage is row-oriented, but there are also column-oriented and
correlation database In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...
s.


Materialized views

Often storage redundancy is employed to increase performance. A common example is storing ''materialized views'', which consist of frequently needed ''external views'' or query results. Storing such views saves the expensive computing them each time they are needed. The downsides of materialized views are the overhead incurred when updating them to keep them synchronized with their original updated database data, and the cost of storage redundancy.


Replication

Occasionally a database employs storage redundancy by database objects replication (with one or more copies) to increase data availability (both to improve performance of simultaneous multiple end-user accesses to the same database object, and to provide resiliency in a case of partial failure of a distributed database). Updates of a replicated object need to be synchronized across the object copies. In many cases, the entire database is replicated.


Virtualization

With
data virtualization Data virtualization is an approach to data management that allows an application to retrieve and manipulate data without requiring technical details about the data, such as how it is formatted at source, or where it is physically located, and can pr ...
, the data used remains in its original locations and real-time access is established to allow analytics across multiple sources. This can aid in resolving some technical difficulties such as compatibility problems when combining data from various platforms, lowering the risk of error caused by faulty data, and guaranteeing that the newest data is used. Furthermore, avoiding the creation of a new database containing personal information can make it easier to comply with privacy regulations. However, with data virtualization, the connection to all necessary data sources must be operational as there is no local copy of the data, which is one of the main drawbacks of the approach.


Security

Database security deals with all various aspects of protecting the database content, its owners, and its users. It ranges from protection from intentional unauthorized database uses to unintentional database accesses by unauthorized entities (e.g., a person or a computer program). Database access control deals with controlling who (a person or a certain computer program) are allowed to access what information in the database. The information may comprise specific database objects (e.g., record types, specific records, data structures), certain computations over certain objects (e.g., query types, or specific queries), or using specific access paths to the former (e.g., using specific indexes or other data structures to access information). Database access controls are set by special authorized (by the database owner) personnel that uses dedicated protected security DBMS interfaces. This may be managed directly on an individual basis, or by the assignment of individuals and
privileges Privilege may refer to: Arts and entertainment * ''Privilege'' (film), a 1967 film directed by Peter Watkins * ''Privilege'' (Ivor Cutler album), 1983 * ''Privilege'' (Television Personalities album), 1990 * ''Privilege (Abridged)'', an alb ...
to groups, or (in the most elaborate models) through the assignment of individuals and groups to roles which are then granted entitlements. Data security prevents unauthorized users from viewing or updating the database. Using passwords, users are allowed access to the entire database or subsets of it called "subschemas". For example, an employee database can contain all the data about an individual employee, but one group of users may be authorized to view only payroll data, while others are allowed access to only work history and medical data. If the DBMS provides a way to interactively enter and update the database, as well as interrogate it, this capability allows for managing personal databases. Data security in general deals with protecting specific chunks of data, both physically (i.e., from corruption, or destruction, or removal; e.g., see physical security), or the interpretation of them, or parts of them to meaningful information (e.g., by looking at the strings of bits that they comprise, concluding specific valid credit-card numbers; e.g., see data encryption). Change and access logging records who accessed which attributes, what was changed, and when it was changed. Logging services allow for a forensic database audit later by keeping a record of access occurrences and changes. Sometimes application-level code is used to record changes rather than leaving this in the database. Monitoring can be set up to attempt to detect security breaches. Therefore, organizations must take database security seriously because of the many benefits it provides. Organizations will be safeguarded from security breaches and hacking activities like firewall intrusion, virus spread, and ransom ware. This helps in protecting the company’s essential information, which cannot be shared with outsiders at any cause.


Transactions and concurrency

Database transactions A database transaction symbolizes a unit of work, performed within a database management system (or similar system) against a database, that is treated in a coherent and reliable way independent of other transactions. A transaction generally repr ...
can be used to introduce some level of fault tolerance and data integrity after recovery from a crash. A database transaction is a unit of work, typically encapsulating a number of operations over a database (e.g., reading a database object, writing, acquiring or releasing a lock, etc.), an abstraction supported in database and also other systems. Each transaction has well defined boundaries in terms of which program/code executions are included in that transaction (determined by the transaction's programmer via special transaction commands). The acronym
ACID In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
describes some ideal properties of a database transaction: atomicity, consistency, isolation, and durability.


Migration

A database built with one DBMS is not portable to another DBMS (i.e., the other DBMS cannot run it). However, in some situations, it is desirable to migrate a database from one DBMS to another. The reasons are primarily economical (different DBMSs may have different total costs of ownership or TCOs), functional, and operational (different DBMSs may have different capabilities). The migration involves the database's transformation from one DBMS type to another. The transformation should maintain (if possible) the database related application (i.e., all related application programs) intact. Thus, the database's conceptual and external architectural levels should be maintained in the transformation. It may be desired that also some aspects of the architecture internal level are maintained. A complex or large database migration may be a complicated and costly (one-time) project by itself, which should be factored into the decision to migrate. This is in spite of the fact that tools may exist to help migration between specific DBMSs. Typically, a DBMS vendor provides tools to help import databases from other popular DBMSs.


Building, maintaining, and tuning

After designing a database for an application, the next stage is building the database. Typically, an appropriate general-purpose DBMS can be selected to be used for this purpose. A DBMS provides the needed user interfaces to be used by database administrators to define the needed application's data structures within the DBMS's respective data model. Other user interfaces are used to select needed DBMS parameters (like security related, storage allocation parameters, etc.). When the database is ready (all its data structures and other needed components are defined), it is typically populated with initial application's data (database initialization, which is typically a distinct project; in many cases using specialized DBMS interfaces that support bulk insertion) before making it operational. In some cases, the database becomes operational while empty of application data, and data are accumulated during its operation. After the database is created, initialized and populated it needs to be maintained. Various database parameters may need changing and the database may need to be tuned ( tuning) for better performance; application's data structures may be changed or added, new related application programs may be written to add to the application's functionality, etc.


Backup and restore

Sometimes it is desired to bring a database back to a previous state (for many reasons, e.g., cases when the database is found corrupted due to a software error, or if it has been updated with erroneous data). To achieve this, a backup operation is done occasionally or continuously, where each desired database state (i.e., the values of its data and their embedding in database's data structures) is kept within dedicated backup files (many techniques exist to do this effectively). When it is decided by a database administrator to bring the database back to this state (e.g., by specifying this state by a desired point in time when the database was in this state), these files are used to restore that state.


Static analysis

Static analysis techniques for software verification can be applied also in the scenario of query languages. In particular, the *
Abstract interpretation In computer science, abstract interpretation is a theory of sound approximation of the semantics of computer programs, based on monotonic functions over ordered sets, especially lattices. It can be viewed as a partial execution of a computer prog ...
framework has been extended to the field of query languages for relational databases as a way to support sound approximation techniques. The semantics of query languages can be tuned according to suitable abstractions of the concrete domain of data. The abstraction of relational database systems has many interesting applications, in particular, for security purposes, such as fine-grained access control, watermarking, etc.


Miscellaneous features

Other DBMS features might include: * Database logs – This helps in keeping a history of the executed functions. * Graphics component for producing graphs and charts, especially in a data warehouse system. *
Query optimizer Query optimization is a feature of many relational database management systems and other databases such as NoSQL and graph databases. The query optimizer attempts to determine the most efficient way to execute a given query by considering the pos ...
– Performs query optimization on every query to choose an efficient ''
query plan In general, a query is a form of questioning, in a line of inquiry. Query may also refer to: Computing and technology * Query, a precise request for information retrieval made to a database or information system ** Query language, a computer lan ...
'' (a partial order (tree) of operations) to be executed to compute the query result. May be specific to a particular storage engine. * Tools or hooks for database design, application programming, application program maintenance, database performance analysis and monitoring, database configuration monitoring, DBMS hardware configuration (a DBMS and related database may span computers, networks, and storage units) and related database mapping (especially for a distributed DBMS), storage allocation and database layout monitoring, storage migration, etc. Increasingly, there are calls for a single system that incorporates all of these core functionalities into the same build, test, and deployment framework for database management and source control. Borrowing from other developments in the software industry, some market such offerings as " DevOps for database".


Design and modeling

The first task of a database designer is to produce a conceptual data model that reflects the structure of the information to be held in the database. A common approach to this is to develop an entity–relationship model, often with the aid of drawing tools. Another popular approach is the Unified Modeling Language. A successful data model will accurately reflect the possible state of the external world being modeled: for example, if people can have more than one phone number, it will allow this information to be captured. Designing a good conceptual data model requires a good understanding of the application domain; it typically involves asking deep questions about the things of interest to an organization, like "can a customer also be a supplier?", or "if a product is sold with two different forms of packaging, are those the same product or different products?", or "if a plane flies from New York to Dubai via Frankfurt, is that one flight or two (or maybe even three)?". The answers to these questions establish definitions of the terminology used for entities (customers, products, flights, flight segments) and their relationships and attributes. Producing the conceptual data model sometimes involves input from business processes, or the analysis of workflow in the organization. This can help to establish what information is needed in the database, and what can be left out. For example, it can help when deciding whether the database needs to hold historic data as well as current data. Having produced a conceptual data model that users are happy with, the next stage is to translate this into a
schema The word schema comes from the Greek word ('), which means ''shape'', or more generally, ''plan''. The plural is ('). In English, both ''schemas'' and ''schemata'' are used as plural forms. Schema may refer to: Science and technology * SCHEMA ...
that implements the relevant data structures within the database. This process is often called logical database design, and the output is a logical data model expressed in the form of a schema. Whereas the conceptual data model is (in theory at least) independent of the choice of database technology, the logical data model will be expressed in terms of a particular database model supported by the chosen DBMS. (The terms ''data model'' and ''database model'' are often used interchangeably, but in this article we use ''data model'' for the design of a specific database, and ''database model'' for the modeling notation used to express that design). The most popular database model for general-purpose databases is the relational model, or more precisely, the relational model as represented by the SQL language. The process of creating a logical database design using this model uses a methodical approach known as
normalization Normalization or normalisation refers to a process that makes something more normal or regular. Most commonly it refers to: * Normalization (sociology) or social normalization, the process through which ideas and behaviors that may fall outside of ...
. The goal of normalization is to ensure that each elementary "fact" is only recorded in one place, so that insertions, updates, and deletions automatically maintain consistency. The final stage of database design is to make the decisions that affect performance, scalability, recovery, security, and the like, which depend on the particular DBMS. This is often called ''physical database design'', and the output is the physical data model. A key goal during this stage is
data independence Data independence is the type of data transparency that matters for a centralized DBMS. It refers to the immunity of user applications to changes made in the definition and organization of data. Application programs should not, ideally, be expo ...
, meaning that the decisions made for performance optimization purposes should be invisible to end-users and applications. There are two types of data independence: Physical data independence and logical data independence. Physical design is driven mainly by performance requirements, and requires a good knowledge of the expected workload and access patterns, and a deep understanding of the features offered by the chosen DBMS. Another aspect of physical database design is security. It involves both defining
access control In the fields of physical security and information security, access control (AC) is the selective restriction of access to a place or other resource, while access management describes the process. The act of ''accessing'' may mean consuming ...
to database objects as well as defining security levels and methods for the data itself.


Models

A database model is a type of data model that determines the logical structure of a database and fundamentally determines in which manner data can be stored, organized, and manipulated. The most popular example of a database model is the relational model (or the SQL approximation of relational), which uses a table-based format. Common logical data models for databases include: * Navigational databases **
Hierarchical database model A hierarchical database model is a data model in which the data are organized into a tree-like structure. The data are stored as records which are connected to one another through links. A record is a collection of fields, with each field containin ...
** Network model ** Graph database *
Relational model The relational model (RM) is an approach to managing data using a Structure (mathematical logic), structure and language consistent with first-order logic, first-order predicate logic, first described in 1969 by English computer scientist Edgar F. ...
* Entity–relationship model ** Enhanced entity–relationship model * Object model * Document model * Entity–attribute–value model * Star schema An object–relational database combines the two related structures. Physical data models include: * Inverted index * Flat file Other models include: * Multidimensional model * Array model *
Multivalue model A MultiValue database is a type of NoSQL and multidimensional database, typically considered synonymous with PICK, a database originally developed as the Pick operating system. MultiValue databases include commercial products from Rocket Software, ...
Specialized models are optimized for particular types of data: * XML database *
Semantic model A conceptual model is a representation of a system. It consists of concepts used to help people know, understand, or simulate a subject the model represents. In contrast, physical models are physical object such as a toy model that may be assembl ...
*
Content store A content repository or content store is a database of digital content with an associated set of data management, search and access methods allowing application-independent access to the content, rather like a digital library, but with the ability ...
*
Event store {{more citations needed, date=September 2015 An event store is a type of database optimized for storage of events. Conceptually, in an event store, only the ''events'' of a dossier or policy are stored. The idea behind it is that the dossier or ...
* Time series model


External, conceptual, and internal views

A database management system provides three views of the database data: * The external level defines how each group of end-users sees the organization of data in the database. A single database can have any number of views at the external level. * The conceptual level (or ''logical level'') unifies the various external views into a compatible global view. It provides the synthesis of all the external views. It is out of the scope of the various database end-users, and is rather of interest to database application developers and database administrators. * The internal level (or ''physical level'') is the internal organization of data inside a DBMS. It is concerned with cost, performance, scalability and other operational matters. It deals with storage layout of the data, using storage structures such as indexes to enhance performance. Occasionally it stores data of individual views ( materialized views), computed from generic data, if performance justification exists for such redundancy. It balances all the external views' performance requirements, possibly conflicting, in an attempt to optimize overall performance across all activities. While there is typically only one conceptual and internal view of the data, there can be any number of different external views. This allows users to see database information in a more business-related way rather than from a technical, processing viewpoint. For example, a financial department of a company needs the payment details of all employees as part of the company's expenses, but does not need details about employees that are in the interest of the human resources department. Thus different departments need different ''views'' of the company's database. The three-level database architecture relates to the concept of ''data independence'' which was one of the major initial driving forces of the relational model. The idea is that changes made at a certain level do not affect the view at a higher level. For example, changes in the internal level do not affect application programs written using conceptual level interfaces, which reduces the impact of making physical changes to improve performance. The conceptual view provides a level of indirection between internal and external. On the one hand it provides a common view of the database, independent of different external view structures, and on the other hand it abstracts away details of how the data are stored or managed (internal level). In principle every level, and even every external view, can be presented by a different data model. In practice usually a given DBMS uses the same data model for both the external and the conceptual levels (e.g., relational model). The internal level, which is hidden inside the DBMS and depends on its implementation, requires a different level of detail and uses its own types of data structure types.


Research

Database technology has been an active research topic since the 1960s, both in academia and in the research and development groups of companies (for example IBM Research). Research activity includes theory and development of
prototype A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototyp ...
s. Notable research topics have included
models A model is an informative representation of an object, person or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a measure. Models c ...
, the atomic transaction concept, related concurrency control techniques, query languages and query optimization methods, RAID, and more. The database research area has several dedicated academic journals (for example, '' ACM Transactions on Database Systems''-TODS, ''
Data and Knowledge Engineering ''Data & Knowledge Engineering'' is a monthly peer-reviewed academic journal in the area of database systems and knowledge base systems. It is published by Elsevier and was established in 1985. The editor-in-chief is P.P. Chen (Louisiana State Un ...
''-DKE) and annual
conferences A conference is a meeting of two or more experts to discuss and exchange opinions or new information about a particular topic. Conferences can be used as a form of group decision-making, although discussion, not always decisions, are the main pu ...
(e.g.,
ACM ACM or A.C.M. may refer to: Aviation * AGM-129 ACM, 1990–2012 USAF cruise missile * Air chief marshal * Air combat manoeuvring or dogfighting * Air cycle machine * Arica Airport (Colombia) (IATA: ACM), in Arica, Amazonas, Colombia Computing * ...
SIGMOD, ACM PODS, VLDB, IEEE ICDE).


See also

* Comparison of database tools *
Comparison of object database management systems This is a comparison of notable object database management systems, showing what fundamental object database features are implemented natively. See also * Comparison of object–relational database management systems *Comparison of ...
*
Comparison of object–relational database management systems This is a comparison of object–relational database management systems (ORDBMSs). Each system has at least some features of an object–relational database; they vary widely in their completeness and the approaches taken. The following tables c ...
* Comparison of relational database management systems * Data hierarchy * Data bank * Data store * Database theory *
Database testing Database testing usually consists of a layered process, including the user interface (UI) layer, the business layer, the data access layer and the database itself. The UI layer deals with the interface design of the database, while the business l ...
* Database-centric architecture * Flat-file database *
INP (database) INP (pronounced "imp") is a database management system including forms processing data entry. It consists of over fifty utility programs for database access and support, batch updating, and report generation. It was developed by Bob Tidd at the U ...
*
Journal of Database Management The ''Journal of Database Management'' is a top-tier quarterly peer-reviewed academic journal. It was established in 1990 and is published by IGI Global. The editor-in-chief is Professor Keng SIAU (City University of Hong Kong). The scope of the jo ...
* Question-focused dataset


Notes


References


Sources

* * * * * * * * * * * * * * * *


Further reading

* Ling Liu and Tamer M. Özsu (Eds.) (2009).
Encyclopedia of Database Systems
4100 p. 60 illus. . * Gray, J. and Reuter, A. ''Transaction Processing: Concepts and Techniques'', 1st edition, Morgan Kaufmann Publishers, 1992. * Kroenke, David M. and David J. Auer. ''Database Concepts.'' 3rd ed. New York: Prentice, 2007. *
Raghu Ramakrishnan Raghu Ramakrishnan is a researcher in the areas of database and information management. He is a Technical Fellow at Microsoft. He has been a Vice President and Research Fellow for Yahoo! Inc. Ramakrishnan spent 22 years as a professor at the Un ...
and
Johannes Gehrke Johannes Gehrke is a German computer scientist and the director of Microsoft Research in Redmond and CTO and Head of Machine Learning for the Microsoft Teams Backend. He is an ACM Fellow, an IEEE Fellow, and the recipient of the 2011 IEEE Comput ...
,
Database Management Systems
' * Abraham Silberschatz,
Henry F. Korth Henry Francis Korth is a professor of Computer Science and Engineering and co-director of the Computer Science and Business program at Lehigh University. Early life and education Korth holds an A.B degree in mathematics from Williams College ...
, S. Sudarshan,
Database System Concepts
' * * Teorey, T.; Lightstone, S. and Nadeau, T. ''Database Modeling & Design: Logical Design'', 4th edition, Morgan Kaufmann Press, 2005.


External links


DB File extension
nbsp;– information about files with the DB extension {{Authority control