Get Column values as comma seperated string

declare @commaStringList varchar(max)
set @commaStringList = ''
select @commaStringList = @commaStringList + ', ' +
       CAST(ColumnName as VARCHAR) from MyTable 
 

-- remove leading comment
if len(@commaStringList) > 3
  set @commaStringList = substring(@commaStringList, 3, len(@commaStringList) - 2)
 
-- show the result
select @commaStringList