with vrm_ext as (
SELECT   tst_vermieter.vrm_name, tst_extras.ext_name
    FROM tst_vermieter, tst_fahrzeuge, tst_fhz_ext_int, tst_extras
   WHERE (    (tst_vermieter.vrm_id = tst_fahrzeuge.fhz_vrm_id)
          AND (tst_fahrzeuge.fhz_id = tst_fhz_ext_int.fei_fhz_id)
          AND (tst_extras.ext_id = tst_fhz_ext_int.fei_ext_id)
         )
)
select distinct e.vrm_name, e.ext_name, ec.count_extras from 
   vrm_ext e, 
   (select vrm_name, ext_name, count(ext_name) as count_extras from vrm_ext 
    group by vrm_name, ext_name) ec
    where
    ec.vrm_name = e.vrm_name and 
    ec.ext_name = e.ext_name
    order by e.vrm_name