Combine duplicate row data into one row in SQL using commas to separate

1. Define table Variables
Copy codeThe Code is as follows:
DECLARE @ T1 table
(
UserID int,
UserName nvarchar (50 ),
CityName nvarchar (50)
);

Copy codeThe Code is as follows:
Insert into @ T1 (UserID, UserName, CityName) values (1, 'A', 'shanghai ')
Insert into @ T1 (UserID, UserName, CityName) values (2, 'B', 'beijing ')
Insert into @ T1 (UserID, UserName, CityName) values (3, 'C', 'shanghai ')
Insert into @ T1 (UserID, UserName, CityName) values (4, 'D', 'beijing ')
Insert into @ T1 (UserID, UserName, CityName) values (5, 'E', 'shanghai ')

Copy codeThe Code is as follows:
Select * from @ T1
----- Optimal Method
SELECT CityName, STUFF (SELECT ',' + UserName FROM @ T1 subTitle WHERE CityName = A. CityName for xml path (''), 1, 1,'') AS
FROM @ T1
Group by CityName
---- Method 2
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
Group by CityName
) B
Stuff (select ',' + fieldname from tablename for xml path (''), 1, 1 ,'')

This entire sentence is used to concatenate the content of multiple fieldname fields and separate them with commas.
For xml path is a method supported by SQL Server 2005 and later versions to generate XML.
The purpose of the stuff function is to remove the comma separator at the top of the string.
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