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  • Oracle 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

    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;
    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%')
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  • 原文地址:https://www.cnblogs.com/houqijun/p/3373898.html
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