Conditions are used to choose a subset of the rows from a table, or in a control statement such as an IF statement to determine control of flow.
SQL conditions do not follow Boolean logic, where conditions are either true or false. In SQL, every condition evaluates as one of TRUE, FALSE, or UNKNOWN. This is called three-valued logic. The result of a comparison is UNKNOWN if either value being compared is the NULL value.
Rows satisfy a search condition if and only if the result of the condition is TRUE. Rows for which the condition is UNKNOWN do not satisfy the search condition.
Subqueries form an important class of expression that is used in many search conditions.
The different types of search conditions are discussed in the following sections.
You specify a search condition for a WHERE clause, a HAVING clause, a CHECK clause, a JOIN clause, or an IF expression.
{ expression compare expression
| expression compare { ANY | SOME| ALL } ( subquery )
| expression IS [ NOT ] NULL expression
| expression1 IS [ NOT ] DISTINCT FROM expression2
| expression [ NOT ] BETWEEN expression AND expression
| expression [ NOT ] LIKE expression [ ESCAPE expression ]
| expression [ NOT ] IN ( { expression | subquery |
... value-expr1 , value-expr2 [, value-expr3 ] … } )
| column-name [ NOT ] CONTAINS ( … word1 [ , word2, ] [ , word3 ] … )
| CONTAINS ( column-name [ ,...], contains-query string)
| EXISTS ( subquery )
| NOT condition
| condition AND condition
| condition OR condition
| ( condition )
| ( condition , estimate )
| condition IS [ NOT ] { TRUE | FALSE | UNKNOWN }
SELECT Surname, BirthDate FROM Employees WHERE BirthDate <= ALL (SELECT BirthDate FROM Employees);
The subqueries that provide comparison values for quantified comparison predicates might retrieve multiple rows but can have only one column.