9.1. Logical Operators

9.1. Logical Operators
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The usual logical operators are available:

boolean AND boolean → boolean
boolean OR boolean → boolean
NOT boolean → 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.)