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  • SQL条件处理–Case语句

    <Category: SQL Server> 查看评论

    SQL中使用CASE处理条件

         在使用SQL的过程中,有时候可能会要对数据进行条件查询。比如字段A为True时查询出“男”,否则查询出“女”。这样就可以不在程序中在去判断了。在SQL中,可以利用Case语句来完成这样的查询。

    一、Case介绍


    计算条件列表并返回多个可能结果表达式之一。

    CASE 具有两种格式:

    简单 CASE 函数将某个表达式与一组简单表达式进行比较以确定结果。
    CASE 搜索函数计算一组布尔表达式以确定结果。
    两种格式都支持可选的 ELSE 参数。

    语法
    简单 CASE 函数:

    CASE input_expression
        WHEN when_expression THEN result_expression
            [ …n ]
        [
            ELSE else_result_expression
        END

    CASE 搜索函数:

    CASE
        WHEN Boolean_expression THEN result_expression
            [ …n ]
        [
            ELSE else_result_expression
        END

    参数
    input_expression

    是使用简单 CASE 格式时所计算的表达式。Input_expression 是任何有效的 Microsoft® Server™ 表达式。

    WHEN when_expression

    使用简单 CASE 格式时 input_expression 所比较的简单表达式。When_expression 是任意有效的 SQL Server 表达式。Input_expression 和每个 when_expression 的数据类型必须相同,或者是隐性转换。

    n

    占位符,表明可以使用多个 WHEN when_expression THEN result_expression 子句或 WHEN Boolean_expression THEN result_expression 子句。

    THEN result_expression

    当 input_expression = when_expression 取值为 TRUE,或者 Boolean_expression 取值为 TRUE 时返回的表达式。result expression 是任意有效的 SQL Server 表达式。

    ELSE else_result_expression

    当比较运算取值不为 TRUE 时返回的表达式。如果省略此参数并且比较运算取值不为 TRUE,CASE 将返回 NULL 值。Else_result_expression 是任意有效的 SQL Server 表达式。Else_result_expression 和所有 result_expression 的数据类型必须相同,或者必须是隐性转换。

    WHEN Boolean_expression

    使用 CASE 搜索格式时所计算的布尔表达式。Boolean_expression 是任意有效的布尔表达式。

    二、SQL中的Case语句实例

        CASE 函数用于计算多个条件并为每个条件返回单个值。CASE 函数通常的用途是使用可读性更强的值替换代码或缩写。下面的查询使用 CASE 函数重命名书籍的分类,以使之更易理解。

    USE pubs
    SELECT
       CASE type
          WHEN ‘popular_comp’ THEN ‘Popular Computing’
          WHEN ‘mod_cook’ THEN ‘Modern Cooking’
          WHEN ‘business’ THEN ‘Business’
          WHEN ‘psychology’ THEN ‘Psychology’
          WHEN ‘trad_cook’ THEN ‘Traditional Cooking’
          ELSE ‘Not yet categorized’
       END AS Category,
    CONVERT(varchar(30), title) AS “Shortened Title”,
    price AS Price
    FROM titles
    WHERE price IS NOT NULL
    ORDER BY 1

    下面是结果集:

    category            shortened title                Price �
    ——————- —————————— ——-
    Business            Cooking with Computers: Surrep 11.95 �
    Business            Straight Talk About Computers  19.99 �
    Business            The Busy Executive’s Database  19.99 �
    Business            You Can Combat Computer Stress 2.99  �
    Modern Cooking      Silicon Valley Gastronomic Tre 19.99 �
    Modern Cooking      The Gourmet Microwave          2.99  �
    Popular Computing   But Is It User Friendly?       22.95 �
    Popular Computing   Secrets of Silicon Valley      20.00 �
    Psychology          Computer Phobic AND Non-Phobic 21.59 �
    Psychology          Emotional Security: A New Algo 7.99  �
    Psychology          Is Anger the Enemy?            10.95 �
    Psychology          Life Without Fear              7.00  �
    Psychology          Prolonged Data Deprivation: Fo 19.99 �
    Traditional Cooking Fifty Years in Buckingham Pala 11.95 �
    Traditional Cooking Onions, Leeks, and Garlic: Coo 20.95 �
    Traditional Cooking Sushi, Anyone?                 14.99  

    (16 row(s) affected)

    CASE 函数的另一个用途给数据分类。下面的查询使用 CASE 函数对价格分类。

    SELECT
       CASE
          WHEN price IS NULL THEN ‘Not yet priced’
          WHEN price < 10 THEN ‘Very Reasonable Title’
          WHEN price >= 10 and price < 20 THEN ‘Coffee Table Title’
          ELSE ‘Expensive book!’
       END AS “Price Category”,
    CONVERT(varchar(20), title) AS “Shortened Title”
    FROM pubs.dbo.titles
    ORDER BY price

    下面是结果集:

    Price Category        Shortened Title    �
    ——————— ——————–
    Not yet priced        The Psychology of Co
    Not yet priced        Net Etiquette      �
    Very Reasonable Title You Can Combat Compu
    Very Reasonable Title The Gourmet Microwav
    Very Reasonable Title Life Without Fear  �
    Very Reasonable Title Emotional Security:�
    Coffee Table Title    Is Anger the Enemy?�
    Coffee Table Title    Cooking with Compute
    Coffee Table Title    Fifty Years in Bucki
    Coffee Table Title    Sushi, Anyone?     �
    Coffee Table Title    The Busy Executive’s
    Coffee Table Title    Straight Talk About�
    Coffee Table Title    Silicon Valley Gastr
    Coffee Table Title    Prolonged Data Depri
    Expensive book!       Secrets of Silicon V
    Expensive book!       Onions, Leeks, and G
    Expensive book!       Computer Phobic AND�
    Expensive book!       But Is It User Frien

    (18 row(s) affected)

    本文来自: SQL条件处理–Case语句

    
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