MySQL - CASE WHEN的高级用法

Case语法

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    WHEN conditionN THEN resultN
    ELSE result
END;

预制数据

mysql> select * from person;
+----+---------+--------+------+--------+
| id | country | name   | age  | gender |
+----+---------+--------+------+--------+
|  3 | china   | tim    |   10 |      1 |
|  7 | usa     | kobe   |   10 |      1 |
|  8 | usa     | james  |   20 |      1 |
|  9 | usa     | grace  |   30 |      0 |
| 10 | japan   | monica |   40 |      2 |
+----+---------+--------+------+--------+

Case的场景示例

根据字段查询分组结果

mysql> SELECT id, `name`,  CASE gender WHEN 1 THEN '男'  WHEN 0 THEN '女'  ELSE '其他' END AS gender FROM person;
+----+--------+--------+
| id | name   | gender |
+----+--------+--------+
|  3 | tim    | 男     |
|  7 | kobe   | 男     |
|  8 | james  | 男     |
|  9 | grace  | 女     |
| 10 | monica | 其他   |
+----+--------+--------+

分组统计

根据不同国家统计用户的性别

mysql> select country,
    -> sum(case when gender=1 then 1 else 0 end) as male,
    -> sum( case when gender=0 then 1 else 0 end) as female,
    -> sum(case when gender>1 then 1 else 0 end) as unknown
    -> from person
    -> group by country;
+---------+------+--------+---------+
| country | male | female | unknown |
+---------+------+--------+---------+
| china   |    1 |      0 |       0 |
| japan   |    0 |      0 |       1 |
| usa     |    2 |      1 |       0 |
+---------+------+--------+---------+

批量更新

简单Case函数

UPDATE person
SET `status` = CASE id
WHEN 1 THEN 1
WHEN 2 THEN 0
WHEN 3 THEN 0
WHEN 4 THEN 1
END
WHERE id IN (1, 2, 3, 4)

注意事项：一定要有WHERE id IN的限制，否则不在WHEN中的记录就会被置为NULL

Case搜索函数

UPDATE person
SET `status` = CASE
WHEN id%2=1 THEN 1
WHEN id%2=0 THEN 0
END

区别：简单Case函数只能实现相等条件判断，Case搜索函数适合复杂条件判断，比如大于、小于等