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