9.1. Logical Operators
The usual logical operators are available:
booleanANDboolean→booleanbooleanORboolean→booleanNOTboolean→boolean
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 operands
without affecting the result. (However, it is not guaranteed that
the left operand is evaluated before the right operand. See
Section 4.2.14
for more information about the
order of evaluation of subexpressions.)