MySQL 5.1 英文手册 :: 11.2.4 Logical Operators

MySQL 5.1 Reference Manual :: 11 Functions and Operators :: 11.2 Operators :: 11.2.4 Logical Operators
11.2.4. Logical Operators

Table 11.4. 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.

Note that MySQL evaluates any nonzero or 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 && 1;
        -> 1
mysql> SELECT 1 && 0;
        -> 0
mysql> SELECT 1 && NULL;
        -> NULL
mysql> SELECT 0 && NULL;
        -> 0
mysql> 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;
        -> 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 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).

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

MySQL 实验室 因为专注，所以专业。

11.2.4. Logical Operators

MySQL 实验室因为专注，所以专业。