ORDER BY取决于参数导致错误

Question

我有一个存储过程,它依赖于一个参数来启动一个命令:

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......如果我这样调用这个程序:

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东西转换为日期/时间.有人可以帮忙吗？

Answer 1

问题可能是从case. 当使用order by这种方式时，然后使用多个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

问题在于，它case具有单一的输出类型，该类型是在编译查询时确定的。该类型基于组合子句中所有类型的逻辑THEN。因此，每个子句的结果then都会转换为整体类型——这就是错误发生的地方。

您可以在此处阅读有关数据优先级规则的信息。但解决方案很简单：使用多个case语句。