Select (SQL): Difference between revisions

Revision as of 00:52, 12 June 2024

Revision as of 00:52, 12 June 2024

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{{Short description|SQL statement that returns a result set of rows from one or more tables}}

The [[SQL]] '''SELECT''' statement returns a [[result set]] of rows, from one or more [[Table (database)|tables]].<ref>{{cite web |url=http://msdn2.microsoft.com/en-us/library/ms189499.aspx |title=Transact-SQL Syntax Conventions |author=Microsoft |date=23 May 2023 }}</ref><ref>{{cite web |url=http://dev.mysql.com/doc/refman/5.0/en/select.html|title=SQL SELECT Syntax |author=MySQL}}</ref>

The [[SQL]] '''SELECT''' statement returns a [[result set]] of rows, from one or specify how to calculate it. The database translates the query into a "[[query plan]]" which may vary between executions, database versions and database software. This functionality is called the "[[query optimizer]]" as it is responsible for finding the best possible execution plan for the query, within applicable constraints.

TheA SELECT statement retrieves zero or more rows from one or more [[SQLDatabase Tables|database tables]] '''or database [[View (database)|views]]. In most applications, <code>SELECT'''</code> statementis the most commonly used [[data manipulation language]] (DML) command. As SQL returnsis a [[resultdeclarative setprogramming]] oflanguage, rows<code>SELECT</code> queries specify a result set, frombut onedo ornot specify how to calculate it. The database translates the query into a "[[query plan]]" which may vary between executions, database versions and database software. This functionality is called the "[[query optimizer]]" as it is responsible for finding the best possible execution plan for the query, within applicable constraints.

The SELECT statement has many optional clauses:

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|7= the HAVING clause is evaluated for groups for which the HAVING clause is true and inserted into vTable6. For example:

<syntaxhighlight lang="postgresql">

select g.GroupName, count(g.*) as NumberOfMembers

select relational).

from users u inner join groups g on g.Userid = u.Userid

group by GroupName

having count(g.*) > 5

</syntaxhighlight>

|8= the SELECT list is evaluated and returned as Vtable 7

|9= the DISTINCT clause is evaluated; duplicate rows are removed and returned as Vtable 8

|10= the ORDER BY clause is evaluated, ordering the rows and returning VCursor9. This is a cursor and not a table because ANSI defines a cursor as an ordered set of rows (not relational).

}}