Logical Operators
The usual logical operators are available for boolean values:
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.