PostgreSQL 9.2.3 Documentation
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# 9.1. Logical Operators

The usual logical operators are available:

`AND`
`OR`
`NOT`

SQL uses a three-valued logic system with true, false, and `null`, which 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

`a` | NOT `a`
--- | ---
TRUE | FALSE
FALSE | TRUE
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 4.2.14 for more information about the order of evaluation of subexpressions.

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