sql里将重复行数据合并为一行，数据用逗号分隔

 一.定义表变量

 DECLARE @T1 table
 (
 UserID int ,
 UserName nvarchar(50),
 CityName nvarchar(50)
 );

 insert into @T1 (UserID,UserName,CityName) values (1,'a','上海')
 insert into @T1 (UserID,UserName,CityName) values (2,'b','北京')
 insert into @T1 (UserID,UserName,CityName) values (3,'c','上海')
 insert into @T1 (UserID,UserName,CityName) values (4,'d','北京')
 insert into @T1 (UserID,UserName,CityName) values (5,'e','上海')

 select * from @T1

 -----最优的方式
 SELECT CityName,STUFF((SELECT ',' + UserName FROM @T1 subTitle WHERE CityName=A.CityName FOR XML PATH('')),1, 1, '') AS A
 FROM @T1 A
 GROUP BY CityName

 ----第二种方式
 SELECT B.CityName,LEFT(UserList,LEN(UserList)-1)
 FROM (
   SELECT CityName,(SELECT UserName+',' FROM @T1 WHERE CityName=A.CityName FOR XML PATH('')) AS UserList
   FROM @T1 A
   GROUP BY CityName
      ) B

 stuff(select ',' + fieldname  from tablename for xml path('')),1,1,'')
 这一整句的作用是将多行fieldname字段的内容串联起来，用逗号分隔。

 for xml path是SQL Server 2005以后版本支持的一种生成XML的方式。

 stuff函数的作用是去掉字符串最前面的逗号分隔符。