sql里将重复行数据合并为一行，数据用逗号分隔

create table aa
  (
  UserID int ,
  UserName nvarchar(50),
  CityName nvarchar(50)
  );
 
 insert into aa (UserID,UserName,CityName) values (1,'a','上海')
 insert into aa (UserID,UserName,CityName) values (2,'b','北京')
 insert into aa (UserID,UserName,CityName) values (3,'c','上海')
 insert into aa (UserID,UserName,CityName) values (4,'d','北京')
 insert into aa (UserID,UserName,CityName) values (5,'e','上海')
 
 select * from aa

 
 sqlserver
 SELECT CityName,STUFF((SELECT ',' + UserName FROM aa subTitle WHERE CityName=A.CityName FOR XML PATH('')),1, 1, '') AS A
 FROM aa A
 GROUP BY CityName

oracle:

SELECT CityName,LISTAGG(UserName, ',') WITHIN GROUP(ORDER BY UserName) AS usernames
     FROM aa GROUP BY CityName;