sql server: how to concatenate column data using comma

 DECLARE @SQL AS VARCHAR(8000)
SELECT @SQL = ISNULL(@SQL+',','') + ColumnName FROM TableName
SELECT @SQL

DECLARE @CarList nvarchar(max);
SET @CarList = N'';
SELECT @CarList+=CarName+N','
FROM dbo.CARS;
SELECT LEFT(@CarList,LEN(@CarList)-1);

declare @aa varchar (200)
set @aa = ''

select @aa = 
    case when @aa = ''
    then CarName
    else @aa + coalesce(',' + CarName, '')
    end
  from Cars

print @aa