Logical Operators

In SQL, all logical operators evaluate to TRUE, FALSE or NULL (UNKNOWN). In MySQL, this is implemented as 1 (TRUE), 0 (FALSE), and NULL. Most of this is common between different SQL databases, however some may return any non-zero value for TRUE.

NOT , ! Logical NOT. Evaluates to 1 if the operand is 0, to 0 if the operand is non-zero, and NOT NULL returns NULL.
    mysql> SELECT NOT 10;
            -> 0
    mysql> SELECT NOT 0;
            -> 1
    mysql> SELECT NOT NULL;
            -> NULL
    mysql> SELECT ! (1+1);
            -> 0
    mysql> SELECT ! 1+1;
            -> 1
    
The last example produces 1 because the expression evaluates the same way as (!1)+1.
AND , && Logical AND. Evaluates to 1 if all operands are non-zero and not NULL, to 0 if one or more operands are 0, otherwise NULL is returned.
    mysql> SELECT 1 && 1;
            -> 1
    mysql> SELECT 1 && 0;
            -> 0
    mysql> SELECT 1 && NULL;
            -> NULL
    mysql> SELECT 0 && NULL;
            -> 0
    mysql> SELECT NULL && 0;
            -> 0
    
Please note that MySQL versions prior to 4.0.5 stop evaluation when a NULL is encountered, rather than continuing the process to check for possible 0 values. This means that in these versions, SELECT (NULL AND 0) returns NULL instead of 0. In 4.0.5 the code has been re-engineered so that the result will always be as prescribed by the SQL standards while still using the optimization wherever possible.
OR , || Logical OR. Evaluates to 1 if any operand is non-zero, to NULL if any operand is NULL, otherwise 0 is returned.
    mysql> SELECT 1 || 1;
            -> 1
    mysql> SELECT 1 || 0;
            -> 1
    mysql> SELECT 0 || 0;
            -> 0
    mysql> SELECT 0 || NULL;
            -> NULL
    mysql> SELECT 1 || NULL;
            -> 1
    
XOR Logical XOR. Returns NULL if either operand is NULL. For non-NULL operands, evaluates to 1 if an odd number of operands is non-zero, otherwise 0 is returned.
     example_for_help_topic XOR
    mysql> SELECT 1 XOR 1;
            -> 0
    mysql> SELECT 1 XOR 0;
            -> 1
    mysql> SELECT 1 XOR NULL;
            -> NULL
    mysql> SELECT 1 XOR 1 XOR 1;
            -> 1
    
a XOR b is mathematically equal to (a AND (NOT b)) OR ((NOT a) and b). XOR was added in version 4.0.2.