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  • Case用法

    SELECT <myColumnSpec> = 
    CASE 
    WHEN <A> THEN <somethingA> 
    WHEN <B> THEN <somethingB> 
    ELSE <somethingE> 
    END  
     
    在上面的代码中需要用具体的参数代替尖括号中的内容。下面是一个简单的例子: 
     
    USE pubs 
    GO 
    SELECT 
        Title, 
        'Price Range' = 
        CASE 
            WHEN price IS NULL THEN 'Unpriced' 
            WHEN price < 10 THEN 'Bargain' 
            WHEN price BETWEEN 10 and 20 THEN 'Average' 
            ELSE 'Gift to impress relatives' 
        END 
    FROM titles 
    ORDER BY price 
    GO  
     
    这是 CASE 的典型用法,但是使用 CASE 其实可以做更多的事情。比方说下面的 GROUP BY 子句中的 CASESELECT 'Number of Titles', Count(*) 
    FROM titles 
    GROUP BY 
        CASE 
            WHEN price IS NULL THEN 'Unpriced' 
            WHEN price < 10 THEN 'Bargain' 
            WHEN price BETWEEN 10 and 20 THEN 'Average' 
            ELSE 'Gift to impress relatives' 
        END 
    GO  
     
    你甚至还可以组合这些选项,添加一个 ORDER BY 子句,如下所示: 
     
    USE pubs 
    GO 
    SELECT 
        CASE 
            WHEN price IS NULL THEN 'Unpriced' 
            WHEN price < 10 THEN 'Bargain' 
            WHEN price BETWEEN 10 and 20 THEN 'Average' 
            ELSE 'Gift to impress relatives' 
        END AS Range, 
        Title 
    FROM titles 
    GROUP BY 
        CASE 
            WHEN price IS NULL THEN 'Unpriced' 
            WHEN price < 10 THEN 'Bargain' 
            WHEN price BETWEEN 10 and 20 THEN 'Average' 
            ELSE 'Gift to impress relatives' 
        END, 
        Title 
    ORDER BY 
        CASE 
            WHEN price IS NULL THEN 'Unpriced' 
            WHEN price < 10 THEN 'Bargain' 
            WHEN price BETWEEN 10 and 20 THEN 'Average' 
            ELSE 'Gift to impress relatives' 
        END, 
        Title 
    GO  
     
    注意,为了在 GROUP BY 块中使用 CASE,查询语句需要在 GROUP BY 块中重复 SELECT 块中的 CASE 块。 
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  • 原文地址:https://www.cnblogs.com/canyangfeixue/p/3203902.html
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