The usual logical operators are AND, OR, and NOT.

SQL uses a three-valued Boolean logic where the null value represents "unknown". For more information, see the following truth tables.

Table 1. AND/OR truth table
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
Table 2. NOT truth table
a NOT a
True False
False True
Null Null

The operators AND and OR are commutative. You can switch the left and right operand without affecting the result.