Select (SQL): Difference between revisions

Revision as of 14:09, 27 November 2020

Revision as of 07:42, 23 December 2020

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The [[SQL]] '''SELECT''' statement returns a [[result set]] of records, 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 |accessdate= |author=Microsoft }}</ref><ref>{{cite web |url=http://dev.mysql.com/doc/refman/5.0/en/select.html|title=SQL SELECT Syntax |accessdate= |author=MySQL}}</ref>

A SELECT statement retrieves zero or more rows from one or more [[Database Tables|database tables]] or database [[View (database)|views]]. In most applications, <code>SELECT</code> is the most commonly used [[data manipulation language]] (DML) command. As SQL is a [[declarative programming]] language, <code>SELECT</code> queries specify a result set, but do not 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.

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== Overview ==

<code>SELECT</code> is the most common operation in SQL, called "the query". <code>SELECT</code> retrieves data from one or more [[Table (database)|table]]s, or expressions. Standard <code>SELECT</code> statements have no persistent effects on the database. Some non-standard implementations of <code>SELECT</code> can have persistent effects, such as the <code>SELECT INTO</code> syntax provided in some databases.<ref name="ms-sql-select-into">{{ cite book | chapter = Transact-SQL Reference | title = SQL Server Language Reference | series = SQL Server 2005 Books Online | publisher = Microsoft | date = 2007-09-15 | url = http://msdn.microsoft.com/en-us/library/ms188029.aspx | accessdateaccess-date = 2007-06-17 }}</ref>

Queries allow the user to describe desired data, leaving the [[Database management system|database management system (DBMS)]] to carry out [[query plan|planning]], [[query optimizer|optimizing]], and performing the physical operations necessary to produce that result as it chooses.

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| page = 248

| isbn = 9781612905686

| accessdateaccess-date = 2015-10-21

| quote = Although the UNIQUE argument is identical to DISTINCT, it is not an ANSI standard.

}}

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| page = 143

| isbn = 9780074637081

| accessdateaccess-date = 2015-10-21

| quote = [...] the keyword DISTINCT [...] eliminates the duplicates from the result set.

}}

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| <code>SELECT '''TOP 20, 10''' * FROM T</code>

| [[Virtuoso Universal Server|OpenLink Virtuoso]] (skips 20, delivers next 10)<ref name="docs_9.19">{{Cite web |title=9.19.10. The TOP SELECT Option |author=OpenLink Software |work=docs.openlinksw.com |date= |access-date=1 October 2019 |url= http://docs.openlinksw.com/virtuoso/topselectoption/ |language=en-US}}</ref>

|-