如何在查询中使用 CASE 表达式？答案

【问题标题】：How can I use CASE expression in my query?如何在查询中使用 CASE 表达式？
【发布时间】：2019-08-24 18:59:11
【问题描述】：

当我想执行我的 case 语句时，它会显示一些错误。

我创建了一个人口统计查询。所以，我需要包含 case 语句。

SELECT DISTINCT
    'Age' DemographicGroup,
    CASE
        WHEN DATEDIFF(YY, DOB, @ReportDate) 25 THEN '=25' 
        WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 25 AND 35 THEN '26-35' 
        WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 36 AND 45 THEN '36-45' 
        WHEN DATEDIFF(YY, DOB, @ReportDate) 45 THEN '46+'
    END Label, 
    
    CASE
        WHEN DATEDIFF(YY, DOB, @ReportDate) 25 THEN 1 
        WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 25 AND 35 THEN 2 
        WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 36 AND 45 THEN 3 
        WHEN DATEDIFF(YY, DOB, @ReportDate) 45 THEN 4
    END GroupOrder
INTO
    #Demo2
FROM
    [dbo].[Dim_Borrower]

消息 102，第 15 级，状态 1，第 4 行
'25' 附近的语法不正确

消息 102，第 15 级，状态 1，第 7 行
“标签”附近的语法不正确

消息 102，第 15 级，状态 1，第 11 行
“GroupOrder”附近的语法不正确

【问题讨论】：

标签： sql sql-server case

【解决方案1】：

您需要在WHEN 表达式中使用比较运算符。比如=、<、>、<>等

您还应该添加明确的ELSE 大小写以避免无意的NULL 输出：

SELECT DISTINCT
    'Age' DemographicGroup,
    CASE
        WHEN DATEDIFF(YY, DOB, @ReportDate) = 25              THEN '=25' 
        WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 25 AND 35 THEN '26-35' 
        WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 36 AND 45 THEN '36-45' 
        WHEN DATEDIFF(YY, DOB, @ReportDate) > 45              THEN '46+'
        ELSE 'Unknown'
    END AS [Label], 

    CASE
        WHEN DATEDIFF(YY, DOB, @ReportDate) = 25              THEN 1 
        WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 25 AND 35 THEN 2 
        WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 36 AND 45 THEN 3 
        WHEN DATEDIFF(YY, DOB, @ReportDate) > 45              THEN 4
        ELSE 5
    END AS [GroupOrder]
INTO
    #Demo2
FROM
    [dbo].[Dim_Borrower]

您可以通过使用内部查询并使用更简单的CASE xpr WHEN val 而不是更长形式的CASE WHEN expr 来简化此操作以减少重复代码。

SELECT DISTINCT
    'Age' AS DemographicGroup,
    CASE q0.AgeGroup
        WHEN 1 THEN '=25' 
        WHEN 2 THEN '26-35' 
        WHEN 3 THEN '36-45' 
        WHEN 4 THEN '46+'
        ELSE 'Unknown'
    END AS [Label], 
    q0.AgeGroup AS [GroupOrder]
INTO
    #Demo2
FROM
    (
        SELECT
            'Age' AS DemographicGroup,
            CASE
                WHEN DATEDIFF(YY, DOB, @ReportDate) = 25              THEN 1 
                WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 25 AND 35 THEN 2 
                WHEN DATEDIFF(YY, DOB, @ReportDate) BETWEEN 36 AND 45 THEN 3 
                WHEN DATEDIFF(YY, DOB, @ReportDate) > 45              THEN 4
                ELSE 5
            END AS AgeGroup
        FROM
            [dbo].[Dim_Borrower]
    ) AS q0

【讨论】：

【解决方案2】：

APPLY 的案例。你也不需要 BETWEEN 因为 CASE Evaluates, in the order specified, Boolean_expression for each WHEN clause

SELECT DISTINCT
    'Age' DemographicGroup,
    CASE g.[GroupOrder]
        WHEN 1 THEN '=25' 
        WHEN 2 THEN '26-35' 
        WHEN 3 THEN '36-45' 
        WHEN 4 THEN '46+'
        ELSE 'Unknown'
    END AS [Label], 
    g.[GroupOrder]
INTO
    #Demo2
FROM
    [dbo].[Dim_Borrower]
CROSS APPLY (
   SELECT CASE
        WHEN DATEDIFF(YY, DOB, @ReportDate) = 25  THEN 1 
        WHEN DATEDIFF(YY, DOB, @ReportDate) <= 35 THEN 2 
        WHEN DATEDIFF(YY, DOB, @ReportDate) <= 45 THEN 3 
        WHEN DATEDIFF(YY, DOB, @ReportDate) > 45  THEN 4
        ELSE 5
    END AS [GroupOrder]
) g

【讨论】：

【解决方案3】：

我很困惑为什么你指的是任何表格。为什么不直接使用包含所需值的派生表？

select 'Age' as DemographicGroup,
into #Demo2
from (values (1, '=25'),
             (2, '26-35'),
             (3, '36-45'),
             (4, '46+')
     ) v(Label, GroupOrder);

这确保所有值都进入表中，无论Dim_Borrower 是否有它们。

我还将包括年龄范围的下限和上限，以便我可以 join 将此添加到其他数据：

select 'Age' as DemographicGroup,
into #Demo2
from (values (1, '=25', 25, 25),
             (2, '26-35', 26, 35),
             (3, '36-45', 36, 45),
             (4, '46+', 46, NULL)
     ) v(Label, GroupOrder, LowerAge, UpperAge);

【讨论】：

感谢您的帮助。我真的很感谢你们。