sql里将重复行数据合并为一行，数据用逗号分隔

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。