9.1. 邏輯運算子

The usual logical operators are available:

AND
OR
NOT

SQLuses a three-valued logic system with true, false, andnull, 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 operatorsANDandORare commutative, that is, you can switch the left and right operand without affecting the result. But seeSection 4.2.14for more information about the order of evaluation of subexpressions.