1.How to concatenating multiple rows in a single column in Sql Server?

Create [dbo].[GetCartIds](@nuserid VARCHAR(200))

RETURNS VARCHAR(8000)
AS
BEGIN
DECLARE @Output VARCHAR(8000)
SET @Output = ''
SELECT @Output = CASE @Output
WHEN '' THEN convert(varchar(50),nCartId)
ELSE @Output + ',' +convert(varchar(50),nCartId)
END
from tblcart where nuserid=@nuserid
RETURN @Output
END