12.3.3 Logical Operators

Table 12.4 Logical Operators

Name	Description
AND, &&	Logical AND
NOT, !	Negates value
||, OR	Logical OR
XOR	Logical XOR

In SQL, all logical operators evaluate to TRUE, FALSE, or NULL (UNKNOWN). In MySQL, these are implemented as 1 (TRUE), 0 (FALSE), and NULL. Most of this is common to different SQL database servers, although some servers may return any nonzero value for TRUE.

MySQL evaluates any nonzero, non-NULL value to TRUE. For example, the following statements all assess to TRUE:

mysql> SELECT 10 IS TRUE;
-> 1
mysql> SELECT -10 IS TRUE;
-> 1
mysql> SELECT 'string' IS NOT NULL;
-> 1

• NOT, !

  Logical NOT. Evaluates to 1 if the operand is 0, to 0 if the operand is nonzero, 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 nonzero and not NULL, to 0 if one or more operands are 0, otherwise NULL is returned.

  mysql> SELECT 1 AND 1;
          -> 1
  mysql> SELECT 1 AND 0;
          -> 0
  mysql> SELECT 1 AND NULL;
          -> NULL
  mysql> SELECT 0 AND NULL;
          -> 0
  mysql> SELECT NULL AND 0;
          -> 0

• OR, ||

  Logical OR. When both operands are non-NULL, the result is 1 if any operand is nonzero, and 0 otherwise. With a NULL operand, the result is 1 if the other operand is nonzero, and NULL otherwise. If both operands are NULL, the result is NULL.

  mysql> SELECT 1 OR 1;
          -> 1
  mysql> SELECT 1 OR 0;
          -> 1
  mysql> SELECT 0 OR 0;
          -> 0
  mysql> SELECT 0 OR NULL;
          -> NULL
  mysql> SELECT 1 OR 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 nonzero, otherwise 0 is returned.

  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).