SELECT
  *, configured - used as remaining
FROM (SELECT
  COALESCE(SUM(db_units), 0)
    + COALESCE(SUM(vm_units), 0)
    + COALESCE(SUM(filer_units), 0)
    + COALESCE(SUM(normal_units), 0)
    AS used,
  %d AS configured
  FROM backup_unit_overview) subquery_a;
