排序规则引起的冲突问题

最近在工作中碰到一例因排序规则而导致的冲突问题，运行环境是SQL 2008，具体代码如下：

DECLARE @URL VARCHAR(500), @startdate DATETIME, @enddate DATETIME, @Identifier VARCHAR(20); SELECT @URL = '/articlenet/article.aspx', @startdate = '2010-02-01', @enddate = '2010-02-28', @Identifier = 'id'; SELECT CAST(au.PopulateDate AS date) AS DateViewed ,qs.Value ,COUNT(1) AS PageViews ,COUNT( DISTINCT CASE WHEN au.UserId = 6264375 THEN au.FPID ELSE du.UserGUID END ) AS DistinctUsers FROM Dim_URL u (NOLOCK) JOIN dbo.Agg_User_Archive au (NOLOCK) ON u.URLKey = au.URLKey JOIN Dim_QueryString qs (NOLOCK) ON au.AggUserId = qs.AggUserId JOIN dbo.Dim_UserId du (NOLOCK) ON du.UserKey = au.UserId WHERE URL = @URL AND au.PopulateDate > @startdate AND au.PopulateDate < @enddate AND qs.Identifier = @Identifier GROUP BY CAST(au.PopulateDate AS DATE), Value ORDER BY DateViewed DESC, Value; --Msg 457, Level 16, State 1, Line 11 --Implicit conversion of varchar value to varchar cannot be performed because the collation of the value is unresolved due to a collation conflict.  /*该查询是一个已经编写好的查询语句，只是根据需要我们做出了适当的调整。即新增了一个JOIN表Dim_UserId，然后将CASE子句中THEN后   面跟的，原来为FPIDKey更换为FPID，ELSE后面的UserId更换为UserGUID。修改后的批处理中语法检查时并没有发现任何错误。执行时出现  上述错误提示。从错误的提示来分析是因为排序冲突所致，因此查看新增的两个字段是否使用了相同的排序规则。下面是查看语句： */  SELECT o.name,o.object_id,c.name,c.column_id,c.collation_name FROM sys.columns c JOIN sys.objects o ON c.object_id = o.object_id WHERE o.object_id = OBJECT_ID('dbo.Dim_UserId') AND c.name = 'UserGUID' UNION SELECT o.name,o.object_id,c.name,c.column_id,c.collation_name FROM sys.columns c JOIN sys.objects o ON c.object_id = o.object_id WHERE o.object_id = OBJECT_ID('dbo.Agg_User_Archive') AND c.name = 'FPID' /* name object_id name column_id collation_name ------------------ ----------- ---------- ----------- ---------------------- Agg_User_Archive 1613248802 FPID 28 Latin1_General_CS_AI Dim_UserId 1234819461 UserGUID 2 Latin1_General_BIN */ --从查询结果中可以看出，原来是因为两个列使用的不同的排序规则，故在count运算时发生了错误。于是修改语句如下，问题解决。下面仅列出被修改过的语句。 COUNT(DISTINCT CASE WHEN au.UserId = 6264375 THEN au.FPID COLLATE DATABASE_DEFAULT ELSE du.UserGUID COLLATE DATABASE_DEFAULT END ) AS DistinctUsers

以下列出Collate子句的语法：
COLLATE { <collation_name> | database_default }
<collation_name> :: =      { Windows_collation_name } | { SQL_collation_name }

几点注意事项：
一、排序规则可以在以下几个级别指定：
1.创建或更改数据库。
2.创建或更改表列。
3.投影表达式的排序规则。
二、COLLATE 子句只能应用于 char、varchar、text、nchar、nvarchar 和 ntext 数据类型。

其它关于排序规则问题请参照本人的其它文章：SQL server 排序规则（COLLATE）
更多参考：http://msdn.microsoft.com/zh-cn/library/ms184391.aspx