oracle之 case when 的用法

oracle case when的用法

SELECT col1, col2,

CASE

WHEN col3 > 1 AND col3 <2

THEN '1'

WHEN col3 > 2 AND col3 <3

THEN '2'

WHEN col3 > 3 AND col3 <4

THEN '3'

ELSE '4'

END mylevel

FROM table1

注意点：

1、以CASE开头，以END结尾

2、分支中WHEN 后跟条件，THEN为显示结果

3、ELSE 为除此之外的默认情况，类似于高级语言程序中switch case的default，可以不加

4、END 后跟别名

============================================================================================

表达式有两种表达方式：

--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

SELECT CASE WHEN 用法

SELECT   grade, COUNT (CASE WHEN sex = 1 THEN 1      /*sex 1为男生，2位女生*/
ELSE NULL
END) 男生数,
COUNT (CASE WHEN sex = 2 THEN 1
ELSE NULL
END) 女生数
FROM students GROUP BY grade;

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

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;

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。

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。