[sql-server] How to make a query with group_concat in sql server

This can also be achieved using the Scalar-Valued Function in MSSQL 2008
Declare your function as following,

CREATE FUNCTION [dbo].[FunctionName]
(@MaskId INT)
RETURNS Varchar(500) 
AS
BEGIN

    DECLARE @SchoolName varchar(500)                        

    SELECT @SchoolName =ISNULL(@SchoolName ,'')+ MD.maskdetail +', ' 
    FROM maskdetails MD WITH (NOLOCK)       
    AND MD.MaskId=@MaskId

    RETURN @SchoolName

END

And then your final query will be like

SELECT m.maskid,m.maskname,m.schoolid,s.schoolname,
(SELECT [dbo].[FunctionName](m.maskid)) 'maskdetail'
FROM tblmask m JOIN school s on s.id = m.schoolid 
ORDER BY m.maskname ;

Note: You may have to change the function, as I don't know the complete table structure.