1. CASE WHEN 表达式有两种形式

--简单Case函数

CASE sex

WHEN '1' THEN '男'

WHEN '2' THEN '女'

ELSE '其他' END

--Case搜索函数

CASE

WHEN sex = '1' THEN '男'

WHEN sex = '2' THEN '女'

ELSE '其他' END

2. CASE WHEN 在语句中不同位置的用法

2.1 SELECT CASE WHEN 用法

select g.group_id,g.group_name 产品名称,count(case when uo.operation_type = 1 then 1 else null end) 话费支付总数

,count(case when uo.operation_type = 3 then 1 else null end ) 第三方支付总数

from educloud.edu_group g,educloud.edu_user_order_items it,educloud.edu_user_order uo

where g.group_id = it.group_id and it.user_id = uo.user_id and

it.group_id = uo.group_id and it.state in (1,2,3,4) and it.source = 0

and uo.commit_state =1 and uo.operation_type in (1,3) and

uo.source=0 and g.group_id in (5000014,5000015,5000011,5000010,5000017) group by g.group_id,g.group_name;

2.2 WHERE CASE WHEN 用法

SELECT T2.*, T1.*

FROM T1, T2

WHERE (CASE WHEN T2.COMPARE_TYPE = 'A' AND

T1.SOME_TYPE LIKE 'NOTHING%'

THEN 1

WHEN T2.COMPARE_TYPE != 'A' AND

T1.SOME_TYPE NOT LIKE 'NOTHING%'

THEN 1

ELSE 0

END) = 1

2.3 GROUP BY CASE WHEN 用法

SELECT

CASE WHEN salary <= 500 THEN '1'

WHEN salary > 500 AND salary <= 600 THEN '2'

WHEN salary > 600 AND salary <= 800 THEN '3'

WHEN salary > 800 AND salary <= 1000 THEN '4'

ELSE NULL END salary_class, -- 别名命名

COUNT(*)

FROM Table_A

GROUP BY

CASE WHEN salary <= 500 THEN '1'

WHEN salary > 500 AND salary <= 600 THEN '2'

WHEN salary > 600 AND salary <= 800 THEN '3'

WHEN salary > 800 AND salary <= 1000 THEN '4'

ELSE NULL END;

3.关于IF-THEN-ELSE的其他实现

3.1 DECODE() 函数

select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')

from employees;

貌似只有Oracle提供该函数，而且不支持ANSI SQL，语法上也没CASE WHEN清晰，个人不推荐使用。

3.2 在WHERE中特殊实现

SELECT T2.*, T1.*

FROM T1, T2

WHERE (T2.COMPARE_TYPE = 'A' AND T1.SOME_TYPE LIKE 'NOTHING%')

OR

(T2.COMPARE_TYPE != 'A' AND T1.SOME_TYPE NOT LIKE 'NOTHING%')

这种方法也是在特殊情况下使用，要多注意逻辑，不要弄错。