Content deleted Content added
Add SELECT clause, better wording for AS (it doesn't "temporarily rename" columns), move AS after SELECT clause |
Mention the relational operations that match some of the SELECT clauses |
||
Line 5:
The SELECT statement has many optional clauses:
* <code>SELECT</code> clause is the list of columns or SQL expressions that must be returned by the query. This is approximately the [[relational algebra]] [[Projection_(relational_algebra)|projection]] operation.
* <code>[[Alias (SQL)|AS]]</code> optionally provides an alias for each column or expression in the <code>SELECT</code> clause. 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 isn't 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.
| |||