| PostgreSQL 8.2.6 Documentation | ||||
|---|---|---|---|---|
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The usual logical operators are available:
| AND |
| OR |
| NOT |
SQL uses a three-valued Boolean logic where the null value represents "unknown" . Observe the following truth tables:
| a | b | a AND b | a OR b |
|---|---|---|---|
| TRUE | TRUE | TRUE | TRUE |
| TRUE | FALSE | FALSE | TRUE |
| TRUE | NULL | NULL | TRUE |
| FALSE | FALSE | FALSE | FALSE |
| FALSE | NULL | FALSE | NULL |
| NULL | NULL | NULL | NULL |
The operators AND and OR are commutative, that is, you can switch the left and right operand without affecting the result. But see Section 4.2.12 for more information about the order of evaluation of subexpressions.