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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