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Case (SQL)
The syntax of the SQL programming language is defined and maintained by ISO/IEC SC 32 as part of ''ISO/IEC 9075''. This standard is not freely available. Despite the existence of the standard, SQL code is not completely portable among different database systems without adjustments. Language elements , width=500, caption=A chart showing several of the SQL language elements that compose a single statement. This adds one to the population of the USA in the country table. The SQL language is subdivided into several language elements, including: * ''Keywords'' are words that are defined in the SQL language. They are either reserved (e.g. , and ), or non-reserved (e.g. , and ). List of SQL reserved words. * ''Identifiers'' are names on database objects, like tables, columns and schemas. An identifier may not be equal to a reserved keyword, unless it is a delimited identifier. Delimited identifiers means identifiers enclosed in double quotation marks. They can contain characte ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Syntax (programming Languages)
In computer science, the syntax of a computer language is the rules that defines the combinations of symbols that are considered to be correctly structured statements or expressions in that language. This applies both to programming languages, where the document represents source code, and to markup languages, where the document represents data. The syntax of a language defines its surface form. Text-based computer languages are based on sequences of characters, while visual programming languages are based on the spatial layout and connections between symbols (which may be textual or graphical). Documents that are syntactically invalid are said to have a syntax error. When designing the syntax of a language, a designer might start by writing down examples of both legal and illegal strings, before trying to figure out the general rules from these examples. Syntax therefore refers to the ''form'' of the code, and is contrasted with semantics – the ''meaning''. In processi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
SQL CASE
In SQL, null or NULL is a special marker used to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd, SQL null serves to fulfil the requirement that all ''true relational database management systems ( RDBMS)'' support a representation of "missing information and inapplicable information". Codd also introduced the use of the lowercase Greek omega (ω) symbol to represent null in database theory. In SQL, NULL is a reserved word used to identify this marker. A null should not be confused with a value of 0. A null value indicates a lack of a value, which is not the same thing as a value of zero. For example, consider the question "How many books does Adam own?" The answer may be "zero" (we ''know'' that he owns ''none'') or "null" (we ''do not know'' how many he owns). In a database table, the column reporting this answer would start out with no value (marked by Null), and it would not be updated ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Order By (SQL)
An ORDER BY clause in SQL specifies that a SQL SELECT statement returns a result set with the rows being sorted by the values of one or more columns. The sort criteria do not have to be included in the result set. The sort criteria can be expressions, including column names, user-defined functions, arithmetic operations, or CASE expressions. The expressions are evaluated and the results are used for the sorting, i.e., the values stored in the column or the results of the function call. ORDER BY is the ''only'' way to sort the rows in the result set. Without this clause, the relational database system may return the rows in any order. If an ordering is required, the ORDER BY must be provided in the SELECT statement sent by the application. Although some database systems allow the specification of an ORDER BY clause in subselects or view definitions, the presence there has no effect. A view is a logical relational table, and the relational model mandates that a table is a s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Having (SQL)
A HAVING clause in SQL specifies that an SQL SELECT statement must only return rows where aggregate values meet the specified conditions. HAVING and WHERE Where may refer to: * Where?, one of the Five Ws in journalism * where (command), a shell command * Where (SQL), a database language clause * Where.com, a provider of location-based applications via mobile phones * ''Where'' (magazine), a serie ... are often confused by beginners, but they serve different purposes. WHERE is taken into account at an earlier stage of a query execution, filtering the rows read from the tables. If a query contains GROUP BY, rows from the tables are grouped and aggregated. After the aggregating operation, HAVING is applied, filtering out the rows that don't match the specified conditions. Therefore, WHERE applies to data read from tables, and HAVING should only apply to aggregated data, which isn't known in the initial stage of a query. To view the present condition formed by the GROUP BY clau ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Group By (SQL)
A GROUP BY statement in SQL specifies that a SQL SELECT statement partitions result rows into groups, based on their values in one or several columns. Typically, grouping is used to apply some sort of aggregate function for each group. The result of a query using a GROUP BY statement contains one row for each group. This implies constraints on the columns that can appear in the associated SELECT clause. As a general rule, the SELECT clause may only contain columns with a unique value per group. This includes columns that appear in the GROUP BY clause as well as aggregates resulting in one value per group. Examples Returns a list of Department IDs along with the sum of their sales for the date of January 1, 2000. SELECT DeptID, SUM(SaleAmount) FROM Sales WHERE SaleDate = '01-Jan-2000' GROUP BY DeptID Returns the data of the example pivot table which answers the question "How many Units did we sell in each Region for every Ship Date?": SELECT Region, Ship_Date, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Result Set
An SQL result set is a set of rows from a database, as well as metadata about the query such as the column names, and the types and sizes of each column. Depending on the database 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 spa ..., the number of rows in the result set may or may not be known. Usually, this number is not known up front because the result set is built on-the-fly. A result set is effectively a table. The ORDER BY clause can be used in a query to impose a certain sort condition on the rows. Without that clause, there is no guarantee whatsoever on the order in which the rows are returned. Databases {{database-software-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Join (SQL)
A join clause in SQL – corresponding to a join operation in relational algebra – combines columns from one or more tables into a new table. Informally, a join stitches two tables and puts on the same row records with matching fields : INNER, LEFT OUTER, RIGHT OUTER, FULL OUTER and CROSS. Example tables To explain join types, the rest of this article uses the following tables: Department.DepartmentID is the primary key of the Department table, whereas Employee.DepartmentID is a foreign key. Note that in Employee, "Williams" has not yet been assigned to a department. Also, no employees have been assigned to the "Marketing" department. This is the SQL statement to create the above tables: CREATE TABLE department( DepartmentID INT PRIMARY KEY NOT NULL, DepartmentName VARCHAR(20) ); CREATE TABLE employee ( LastName VARCHAR(20), DepartmentID INT REFERENCES department(DepartmentID) ); INSERT INTO department VALUES (31, 'Sales'), (33, 'Engineering') ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
From (SQL)
The SQL From clause is the source of a rowset to be operated upon in a Data Manipulation Language (DML) statement. From clauses are very common, and will provide the rowset to be exposed through a Select statement, the source of values in an Update statement, and the target rows to be deleted in a Delete statement. FROM is an SQL reserved word in the SQL standard. The FROM clause is used in conjunction with SQL statements, and takes the following general form: ''SQL-DML-Statement'' FROM ''table_name'' WHERE ''predicate'' The From clause can generally be anything that returns a rowset, a table, view, function, or system-provided information like the Information Schema, which is typically running proprietary commands and returning the information in a table form. Examples The following query returns only those rows from table ''mytable'' where the value in column ''mycol'' is greater than 100. SELECT * FROM mytable WHERE mycol > 100 Requirement The From c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 possible query plans. Generally, the query optimizer cannot be accessed directly by users: once queries are submitted to the database server, and parsed by the parser, they are then passed to the query optimizer where optimization occurs. However, some database engines allow guiding the query optimizer with hints. A query is a request for information from a database. It can be as simple as "find the address of a person with Social Security number 123-45-6789," or more complex like "find the average salary of all the employed married men in California between the ages 30 to 39 who earn less than their spouses." The result of a query is generated by processing the rows in a database in a way that yields the requested information. Since ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 language used to make queries into databases and information systems ** Query string, in the World Wide Web, is an optional part of a URL ** Web search query, a query entered by users into web search engines * Command-query separation (CQS), a concept in object-oriented programming, especially in the Eiffel programming language * jQuery, a lightweight JavaScript library that emphasizes interaction between JavaScript and HTML People with the name * Jeff Query (born 1967), an American football player * Nate Query, a musician Other uses * Query (complexity), a mapping from structures of one vocabulary to structures of another vocabulary * Query (Quaker), a question used for reflection and spiritual exercises among members of the Socie ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 spans formal techniques and practical considerations, including data modeling, efficient data representation and storage, query languages, security and privacy of sensitive data, and distributed computing issues, including supporting concurrent access and fault tolerance. A database management system (DBMS) is the software that interacts with end users, 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 a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Table (database)
A table is a collection of related data held in a table format within a database. It consists of columns and rows. In relational databases, and flat file databases, a ''table'' is a set of data elements (values) using a model of vertical columns (identifiable by name) and horizontal rows, the cell being the unit where a row and column intersect. A table has a specified number of columns, but can have any number of rows. Each row is identified by one or more values appearing in a particular column subset. A specific choice of columns which uniquely identify rows is called the primary key. "Table" is another term for "relation"; although there is the difference in that a table is usually a multiset (bag) of rows where a relation is a set and does not allow duplicates. Besides the actual data rows, tables generally have associated with them some metadata, such as constraints on the table or on the values within particular columns. The data in a table does not have to b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
