ORDER BY 取决于参数导致错误

Question

我有一个根据参数启动订单的存储过程：

DROP PROCEDURE [dbo].[GetUsersByClusterAndUserName]
  GO
CREATE PROCEDURE [dbo].[GetUsersByClusterAndUserName]
    @SortField [nvarchar] (256) = 'UserName',
    @SortOrder [int] = 0
AS
    SELECT * FROM [User] 
    ORDER BY
    CASE WHEN @SortOrder = 0 THEN
        CASE 
            WHEN @SortField = 'UserName' THEN User_UserName 
            WHEN @SortField = 'LastLoginDate' THEN User_LastLoginDate 
            WHEN @SortField = 'CreationDate' THEN User_CreationDate END
    END ASC,    
    CASE WHEN @SortOrder = 1 THEN
        CASE
            WHEN @SortField = 'UserName' THEN User_UserName 
            WHEN @SortField = 'LastLoginDate' THEN User_LastLoginDate
            WHEN @SortField = 'CreationDate' THEN [User_CreationDate] END
    END DESC
RETURN 0
GO

但是...如果我这样调用程序：

EXEC dbo.GetUsersByClusterAndUserName @SortOrder=1, @SortField='UserName'

我收到以下错误：

Msg 241, Level 16, State 1, Procedure GetUsersByClusterAndUserName, Line 7
Conversion failed when converting date and/or time from character string.

为什么它会尝试将某些内容转换为日期/时间。有人可以帮忙吗？

Answer 1

问题可能是来自case 的类型转换。以这种方式使用order by时，则使用多个case语句：

ORDER BY (CASE WHEN @SortOrder = 0 AND @SortField = 'UserName' THEN User_UserName END),
         (CASE WHEN @SortOrder = 0 AND @SortField = 'User_LastLoginDate' THEN User_LastLoginDate END),
         (CASE WHEN @SortOrder = 0 AND @SortField = 'User_CreationDate' THEN User_CreationDate END),
         (CASE WHEN @SortOrder = 1 AND @SortField = 'UserName' THEN User_UserName END) DESC,
         (CASE WHEN @SortOrder = 1 AND @SortField = 'User_LastLoginDate' THEN User_LastLoginDate END) DESC,
         (CASE WHEN @SortOrder = 1 AND @SortField = 'User_CreationDate' THEN User_CreationDate END) DESC

问题是case 有一个输出类型，在编译查询时确定。此类型基于组合THEN 子句中的所有类型的逻辑。因此，每个 then 子句的结果都将转换为整体类型——这就是您发生错误的地方。

您可以阅读数据优先规则here。但解决方案很简单：使用多个case 语句。