oracle case when的用法

CASE WHEN 在语句中不同位置的用法

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、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

3、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;

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