9.1. Logical Operators
The usual logical operators are available:
AND

OR

NOT

SQL
uses a threevalued 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.