Select (SQL): Difference between revisions

Revision as of 12:29, 26 December 2024

Revision as of 20:59, 25 January 2025

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* <code>[[Alias (SQL)|AS]]</code> optionally provides an alias for each column or expression in the <code>SELECT</code> list. This is the relational algebra [[Rename_(relational_algebra)|rename]] operation.

* <code>[[From (SQL)|FROM]]</code> specifies from which table to get the data.<ref>Omitting FROM clause is not standard, but allowed by most major DBMSes.</ref>

* <code>[[Where (SQL)|WHERE]]</code> specifies which rows to retrieve. This is approximately the relational algebra [[Selection_(relational_algebra)|selection]] operation.

* <code>[[Group by (SQL)|GROUP BY]]</code> groups rows sharing a property so that an [[aggregate function]] can be applied to each group.

* <code>[[Having (SQL)|HAVING]]</code> selects among the groups defined by the GROUP BY clause.

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* The <code>[[From (SQL)|FROM]]</code> clause, which indicates the tables to retrieve data from. The <code>FROM</code> clause can include optional <code>[[Join (SQL)|JOIN]]</code> subclauses to specify the rules for joining tables.

* The <code>[[Where (SQL)|WHERE]]</code> clause includes a comparison predicate, which restricts the rows returned by the query. The <code>WHERE</code> clause eliminates all rows from the result set where the comparison predicate does not evaluate to True.

* The <code>GROUP BY</code> clause projects rows having common values into a smaller set of rows. <code>GROUP BY</code> is often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result set. The <code>WHERE</code> clause is applied before the <code>GROUP BY</code> clause.

* The <code>[[Having (SQL)|HAVING]]</code> clause includes a predicate used to filter rows resulting from the <code>GROUP BY</code> clause. Because it acts on the results of the <code>GROUP BY</code> clause, aggregation functions can be used in the <code>HAVING</code> clause predicate.