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;-> 1mysql> SELECT -10 IS TRUE;-> 1mysql> 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;        -> 0mysql> SELECT NOT 0;        -> 1mysql> SELECT NOT NULL;        -> NULLmysql> SELECT ! (1+1);        -> 0mysql> 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 &&
                    1;        -> 1mysql> SELECT 1 &&
                    0;        -> 0mysql> SELECT 1 &&
                    NULL;        -> NULLmysql> SELECT 0 &&
                    NULL;        -> 0mysql> SELECT NULL &&
                    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 || 1;        -> 1mysql> SELECT 1 || 0;        -> 1mysql> SELECT 0 || 0;        -> 0mysql> SELECT 0 || NULL;        -> NULLmysql> 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 nonzero, otherwise 0 is returned.
```
mysql> SELECT 1 XOR 1;        -> 0mysql> SELECT 1 XOR 0;        -> 1mysql> SELECT 1 XOR NULL;        -> NULLmysql> SELECT 1 XOR 1 XOR 1;        -> 1
```
a XOR b is mathematically equal to (a AND (NOT b)) OR ((NOT a) and b).