Logical Operators

The following logical operators are available: AND, OR and NOT. SQL uses a three-valuad logic system with TRUE, FALSE and NULL. Note that logical operators involving NULL do not always evaluate to NULL. For example, NULL AND FALSE will evaluate to FALSE, and NULL OR TRUE will evaluate to TRUE. Below are the complete 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.