Oracle学习笔记_06_CASE WHEN 用法介绍

1. CASE WHEN 表达式有两种形式

--简单Case函数  

CASE sex
WHEN '' THEN '男'
WHEN '' THEN '女'
ELSE '其他' END  

--Case搜索函数  

CASE
WHEN sex = '' THEN '男'
WHEN sex = '' 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   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;

2.3 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

2.4 GROUP BY CASE WHEN 用法

SELECT
    CASE
        WHEN salary <= 500                   THEN ''
        WHEN salary > 500 AND salary <= 600  THEN ''
        WHEN salary > 600 AND salary <= 800  THEN ''
        WHEN salary > 800 AND salary <= 1000 THEN ''
        ELSE NULL
    END  salary_class, -- 别名命名
    COUNT(*)
FROM    Table_A
GROUP BY
    CASE
        WHEN salary <= 500 THEN ''
        WHEN salary > 500 AND salary <= 600  THEN ''
        WHEN salary > 600 AND salary <= 800  THEN ''
        WHEN salary > 800 AND salary <= 1000 THEN ''
        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。

附录：参考资料

1.Oracle CASE WHEN 用法介绍

2.https://dba.stackexchange.com/questions/1170/oracle-sql-case-in-a-where-clause