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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