Oracle学习笔记_06_CASE WHEN 用法介绍

--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  

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;

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

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;  

select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')
from   employees;

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%')