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) = 12.3 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。
