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.