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9.1. Logical Operators

The usual logical operators are available:

AND
OR
NOT
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 operand without affecting the result. But see Section 4.2.14 for more information about the order of evaluation of subexpressions.