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.