Oracle,CASE,WHEN,用法介绍_介绍的用法
--简单Case函数 CASE sex WHEN '1' THEN '男' WHEN '2' THEN '女' ELSE '其他' END --Case搜索函数 CASEWHEN sex = '1' THEN '男' WHEN sex = '2' THEN '女' ELSE '其他' END
复制代码代码如下: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;
复制代码代码如下: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
复制代码代码如下: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;
复制代码代码如下:
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
复制代码代码如下:
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。