9.1. Logical Operators
Logical Operators
| Operator | Description | Example |
|---|---|---|
| AND | True if both values are true | a AND b |
| OR | True if either value is true | a OR b |
| NOT | True if the value is false | NOT a |
Effect of NULL on Logical Operators
The result of an AND comparison may be NULL if one or both sides of the expression are NULL. If at least one side of an AND operator is FALSE the expression evaluates to FALSE:
SELECT CAST(null AS boolean) AND true; => null
SELECT CAST(null AS boolean) AND false; => false
SELECT CAST(null AS boolean) AND CAST(null AS boolean); => null
The result of an OR comparison may be NULL if one or both sides of the expression are NULL. If at least one side of an OR operator is TRUE the expression evaluates to TRUE:
SELECT CAST(null AS boolean) OR CAST(null AS boolean); => null
SELECT CAST(null AS boolean) OR false; => null
SELECT CAST(null AS boolean) OR true; => true
The following truth table demonstrates the handling of NULL in AND and OR:
| a | b | a AND b | a OR b |
|---|---|---|---|
| TRUE | TRUE | TRUE | TRUE |
| TRUE | FALSE | FALSE | TRUE |
| TRUE | NULL | NULL | TRUE |
| FALSE | TRUE | FALSE | TRUE |
| FALSE | FALSE | FALSE | FALSE |
| FALSE | NULL | FALSE | NULL |
| NULL | TRUE | NULL | TRUE |
| NULL | FALSE | FALSE | NULL |
| NULL | NULL | NULL | NULL |
The logical complement of NULL is NULL as shown in the following example:
SELECT NOT CAST(null AS boolean); => null
The following truth table demonstrates the handling of NULL in NOT:
| a | NOT a |
|---|---|
| TRUE | FALSE |
| FALSE | TRUE |
| NULL | NULL |