SELECT * FROM (
  SELECT *
  FROM backup_unit_overview
  ORDER BY client, fileset
) subq_a
UNION ALL
SELECT 'TOTAL' as client,
       NULL as fileset,
       SUM(db_units) as db_units,
       SUM(vm_units) as vm_units,
       SUM(filer_units) as filer_units,
       SUM(normal_units) as normal_units
FROM backup_unit_overview;
