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 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 '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。
来自：http://www.cnblogs.com/eshizhan/archive/2012/04/06/2435493.html
