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.