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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