sql里将重复行数据合并为一行，数据用逗号分隔

 1 一.定义表变量
 2 
 3 DECLARE @T1 table
 4 (
 5 UserID int , 
 6 UserName nvarchar(50),
 7 CityName nvarchar(50)
 8 );
 9 
10 insert into @T1 (UserID,UserName,CityName) values (1,'a','上海')
11 insert into @T1 (UserID,UserName,CityName) values (2,'b','北京')
12 insert into @T1 (UserID,UserName,CityName) values (3,'c','上海')
13 insert into @T1 (UserID,UserName,CityName) values (4,'d','北京')
14 insert into @T1 (UserID,UserName,CityName) values (5,'e','上海')
15 
16 select * from @T1
17 
18 -----最优的方式
19 SELECT CityName,STUFF((SELECT ',' + UserName FROM @T1 subTitle WHERE CityName=A.CityName FOR XML PATH('')),1, 1, '') AS A
20 FROM @T1 A
21 GROUP BY CityName
22 
23 ----第二种方式
24 SELECT B.CityName,LEFT(UserList,LEN(UserList)-1) 
25 FROM (
26   SELECT CityName,(SELECT UserName+',' FROM @T1 WHERE CityName=A.CityName FOR XML PATH('')) AS UserList
27   FROM @T1 A 
28   GROUP BY CityName
29      ) B
30 
31 stuff(select ',' + fieldname  from tablename for xml path('')),1,1,'')
32 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。
33 
34 for xml path是SQL Server 2005以后版本支持的一种生成XML的方式。
35 
36 stuff函数的作用是去掉字符串最前面的逗号分隔符。