case 用法

CASE case_value
    WHEN when_value THEN statement_list
    [WHEN when_value THEN statement_list] ...
    [ELSE statement_list]
END CASE

将salary表中sex字段中m替换为f，f替换为m

mysql> select * from salary; 
+------+------+------+--------+
| id   | name | sex  | salary |
+------+------+------+--------+
|    1 | A    | m    |   2500 |
|    2 | B    | f    |   1500 |
|    3 | C    | m    |   5500 |
|    4 | D    | f    |    500 |
+------+------+------+--------+
4 rows in set (0.00 sec)


mysql> update salary set sex = case sex when 'm' then 'f' else 'm' end;
Query OK, 4 rows affected (0.00 sec)
Rows matched: 4  Changed: 4  Warnings: 0

mysql> select * from salary;
+------+------+------+--------+
| id   | name | sex  | salary |
+------+------+------+--------+
|    1 | A    | f    |   2500 |
|    2 | B    | m    |   1500 |
|    3 | C    | f    |   5500 |
|    4 | D    | m    |    500 |
+------+------+------+--------+
4 rows in set (0.00 sec)