本节内容：

mysql内置函数case使用介绍

mysql对case函数的解释：

复制代码 代码示例:

mysql> ? case

Many help items for your request exist.

To make a more specific request, please type 'help ',

where is one of the following

topics:

CASE OPERATOR

CASE STATEMENT

mysql> ? case operator

Name: 'CASE OPERATOR'

Description:

Syntax:

CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN

result ...] [ELSE result] END

CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...]

[ELSE result] END

The first version returns the result where value=compare_value. The

second version returns the result for the first condition that is true.

If there was no matching result value, the result after ELSE is

returned, or NULL if there is no ELSE part.

URL: http://dev.mysql.com/doc/refman/5.1/en/control-flow-functions.html

例子:

复制代码 代码示例:

mysql> SELECT CASE 1 WHEN 1 THEN 'one'

->     WHEN 2 THEN 'two' ELSE 'more' END;

-> 'one'

mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;

-> 'true'

mysql> SELECT CASE BINARY 'B'

->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;

-> NULL

例1：

复制代码 代码示例:

mysql> SELECT CASE 1 WHEN 1 THEN 'one'

->     WHEN 2 THEN 'two' ELSE 'more' END;

-> 'one'

如果case后面的表达式和when中的值相等，则返回相对应then后的值，否则返回else的值。

例2：

复制代码 代码示例:

mysql> SELECT CASE BINARY 'B'

->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;

-> NULL

这个例子和上面的类似，只不过没有else值，返回null

例3：

复制代码 代码示例:

mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;

-> 'true'

如果case后面的值为真，返回then值，否则返回else值。

案例：统计各班级中的及格与不及格人数

复制代码 代码示例:

mysql> select * from student;

+----+-------+-------+-------+

| id | class | name  | score |

+----+-------+-------+-------+

|  1 |     1 | name1 |    50 |

|  2 |     1 | name2 |    30 |

|  3 |     2 | name1 |    60 |

|  4 |     1 | name2 |    30 |

|  5 |     2 | name1 |    60 |

|  6 |     1 | name2 |    70 |

|  7 |     2 | name1 |    60 |

|  8 |     1 | name2 |    70 |

|  9 |     2 | name1 |    60 |

| 10 |     3 | name2 |    70 |

| 11 |     2 | name1 |    60 |

| 12 |     3 | name2 |    20 |

| 13 |     2 | name1 |    60 |

| 14 |     3 | name2 |    20 |

+----+-------+-------+-------+

14 rows in set (0.00 sec)

SQL语句：

复制代码 代码示例:

mysql> select class,count(case when score>=60 then 1 end) as '及格人数',count(case when score<60 then 1 end) as '不及格人数',count(*) as '总人数' from student group by class;

+-------+--------------+-----------------+-----------+

| class | 及格人数     | 不及格人数      | 总人数    |

+-------+--------------+-----------------+-----------+

|     1 |            2 |               3 |         5 |

|     2 |            6 |               0 |         6 |

|     3 |            1 |               2 |         3 |

+-------+--------------+-----------------+-----------+

3 rows in set (0.00 sec)

以上通过实例介绍了mysql内置函数case的用法，希望对大家有所帮助。