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Primary Key
In the relational model of databases, a primary key is a designated attribute (column) that can reliably identify and distinguish between each individual record in a table. The database creator can choose an existing unique attribute or combination of attributes from the table (a natural key) to act as its primary key, or create a new attribute containing a unique ID that exists solely for this purpose (a surrogate key). Examples of natural keys that could be suitable primary keys include data that is already by definition unique to all items in the table such as a national identification number attribute for person records, or the combination of a very precise timestamp attribute with a very precise location attribute for event records. More formally, a primary key is a specific choice of a minimal set of attributes that uniquely specify a tuple ( row) in a relation (table). A primary key is a choice of a candidate key (a minimal superkey); any other candidate key is an ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Relational Model
The relational model (RM) is an approach to managing data using a structure and language consistent with first-order predicate logic, first described in 1969 by English computer scientist Edgar F. Codd, where all data are represented in terms of tuples, grouped into relations. A database organized in terms of the relational model is a relational database. The purpose of the relational model is to provide a declarative method for specifying data and queries: users directly state what information the database contains and what information they want from it, and let the database management system software take care of describing data structures for storing the data and retrieval procedures for answering queries. Most relational databases use the SQL data definition and query language; these systems implement what can be regarded as an engineering approximation to the relational model. A '' table'' in a SQL database schema corresponds to a predicate variable; the contents of a tab ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
2003
2003 was designated by the United Nations as the International Year of Fresh water, Freshwater. In 2003, a Multi-National Force – Iraq, United States-led coalition 2003 invasion of Iraq, invaded Iraq, starting the Iraq War. Demographics The world population on January 1, 2003, was estimated to be 6.272 billion people and increased to 6.353 billion people by January 1, 2004. An estimated 134.0 million births and 52.5 million deaths took place in 2003. The average global life expectancy was 67.1 years, an increase of 0.3 years from 2002. The rate of child mortality was 6.85%, a decrease of 0.27Percentage point, pp from 2002. 25.54% of people were living in extreme poverty, a decrease of 1.31pp from 2002. There were approximately 10.6 million global refugees at the beginning of 2003, and the number was reduced to 9.7 million refugees by the end of the year. Afghanistan was the largest source of refugees, with a total of 2.1 million at th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Strongly Typed Identifier
A strongly typed identifier is user-defined data type which serves as an identifier or key that is Strong and weak typing, strongly typed. This is a solution to the "primitive obsession" code smell as mentioned by Martin Fowler (software engineer), Martin Fowler. The data type should preferably be Immutable object, immutable if possible. It is common for implementations to handle equality testing, serialization and model binding. The strongly typed identifier commonly wraps the data type used as the primary key in the database, such as a string, an integer or universally unique identifier (UUID). Web frameworks can often be configured to model bind properties on view models that are strongly typed identifiers. Object–relational mapping, Object–relational mappers can often be configured with value converters to map data between the properties on a model using strongly typed identifier data types and database columns. Examples Passing a strongly typed identifier throughout the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Entity–relationship Model
An entity–relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between entities (instances of those entity types). In software engineering, an ER model is commonly formed to represent things a business needs to remember in order to perform business processes. Consequently, the ER model becomes an abstract data model, that defines a data or information structure that can be implemented in a database, typically a relational database. Entity–relationship modeling was developed for database and design by Peter Chen and published in a 1976 paper, with variants of the idea existing previously. Today it is commonly used for teaching students the basics of database structure. Some ER models show super and subtype entities connected by generalization-specialization relationships, and an ER model can ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Relational Database
A relational database (RDB) is a database based on the relational model of data, as proposed by E. F. Codd in 1970. A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured format using rows and columns. Many relational database systems are equipped with the option of using SQL (Structured Query Language) for querying and updating the database. History The concept of relational database was defined by E. F. Codd at IBM in 1970. Codd introduced the term ''relational'' in his research paper "A Relational Model of Data for Large Shared Data Banks". In this paper and later papers, he defined what he meant by ''relation''. One well-known definition of what constitutes a relational database system is composed of Codd's 12 rules. However, no commercial implementations of the relational model conform to all of Codd's rules, so the term has gradually come to describe a broader class of database systems, which at a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Composite Key
In database design, a composite key is a candidate key that consists of two or more attributes, (table columns) that together uniquely identify an entity occurrence (table row). A compound key is a composite key for which each attribute that makes up the key is a foreign key in its own right. Advantages Composite keys have advantages similar to that of a natural key as it is often composed of multiple natural key attributes. Storage Composite keys use less disk space as compared to defining a surrogate key column, this is because the composite key already exists as attributes in the table and does not need to be defined in the table just for the purpose of unique identification. This simplifies the table and also saves space. Easier to implement and use Composite keys are easy to implement in a database schema as their component parts are already named items in the database. When they are also natural keys, they are often intuitive for real world scenarios. They are ofte ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Candidate Key
A candidate key, or simply a key, of a relational database is any set of columns that have a unique combination of values in each row, with the additional constraint that removing any column could produce duplicate combinations of values. A candidate key is a minimal superkey, i.e., a superkey that does not contain a smaller one. Therefore, a relation can have multiple candidate keys, each with a different number of attributes. Specific candidate keys are sometimes called ''primary keys'', ''secondary keys'' or ''alternate keys''. The columns in a candidate key are called prime attributes, and a column that does not occur in any candidate key is called a non-prime attribute. Every relation without NULL values will have at least one candidate key: Since there cannot be duplicate rows, the set of all columns is a superkey, and if that is not minimal, some subset of that will be minimal. There is a functional dependency from the candidate key to all the attributes in the relatio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Unique Key
In relational database management systems, a unique key is a candidate key. All the candidate keys of a relation can uniquely identify the records of the relation, but only one of them is used as the primary key of the relation. The remaining candidate keys are called unique keys because they can uniquely identify a record in a relation. Unique keys can consist of multiple columns. Unique keys are also called alternate keys. Unique keys are an alternative to the primary key of the relation. In SQL, the unique keys have a UNIQUE constraint assigned to them in order to prevent duplicates (a duplicate entry is not valid in a unique column). Alternate keys may be used like the primary key when doing a single-table select or when filtering in a ''where'' clause, but are not typically used to join multiple tables. Summary Keys provide the means for database users and application software to identify, access and update information in a database table. ''There may be several keys in any ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Superkey
In the relational data model a superkey is any set of attributes that uniquely identifies each tuple of a relation. Because superkey values are unique, tuples with the same superkey value must also have the same non-key attribute values. That is, non-key attributes are functionally dependent on the superkey. The set of all attributes is always a superkey (the trivial superkey). Tuples in a relation are by definition unique, with duplicates removed after each operation, so the set of all attributes is always uniquely valued for every tuple. A candidate key (or minimal superkey) is a superkey that can't be reduced to a simpler superkey by removing an attribute. For example, in an employee schema with attributes employeeID, name, job, and departmentID, if employeeID values are unique then employeeID combined with any or all of the other attributes can uniquely identify tuples in the table. Each combination, , , , and so on is a superkey. is a candidate key, since no subset of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Natural Key
A natural key (also known as business key or domain key) is a type of unique key in a database formed of attributes that exist and are used in the external world outside the database (i.e. in the business domain or domain of discourse). In the relational model of data, a natural key is a superkey and is therefore a functional determinant for all attributes in a relation. A natural key serves two complementary purposes: * It provides a means of unique identification for data * It imposes a rule, specifically a uniqueness constraint, to ensure that data remains unique within an information system The uniqueness constraint assures uniqueness of data within a certain technical context (e.g. a set of values in a table, file or relation variable) by rejecting input of any data that would otherwise violate the constraint. This means that the user can rely on a guaranteed correspondence between facts identified by key values recorded in a system and the external domain of discourse (a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Due Diligence
Due diligence is the investigation or exercise of care that a reasonable business or person is normally expected to take before entering into an agreement or contract with another party or an act with a certain standard of care. Due diligence can be a legal obligation, but the term more commonly applies to voluntary investigations. It may also offer a defence against legal action. A common example of due diligence is the process through which a potential acquirer evaluates a target company or its assets in advance of a merger or acquisition. The theory behind due diligence holds that performing this type of investigation contributes significantly to informed decision making by enhancing the amount and quality of information available to decision makers and by ensuring that this information is systematically used to deliberate on the decision at hand and all its costs, benefits, and risks. Development of the term The term "due diligence" can be read as "required carefulness" ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Active Record Pattern
In software engineering, the active record pattern is an architectural pattern. It is found in software that stores in-memory object data in relational databases. It was named by Martin Fowler in his 2003 book ''Patterns of Enterprise Application Architecture''. The interface of an object conforming to this pattern would include functions such as Insert, Update, and Delete, plus properties that correspond more or less directly to the columns in the underlying database table. The active record pattern is an approach to accessing data in a database. A database table or view is wrapped into a class. Thus, an object instance is tied to a single row in the table. After creation of an object, a new row is added to the table upon save. Any object loaded gets its information from the database. When an object is updated, the corresponding row in the table is also updated. The wrapper class implements accessor methods or properties for each column in the table or view. This pattern is c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
