SELECT klant, naam, voorl

FROM (

    SELECT klant, COUNT(srtc) AS aantal

    FROM verkoop

       JOIN artikel ON verkoop.art = artikel.art

    WHERE srtc IN (

        SELECT DISTINCT srtc 

        FROM artikel 

           JOIN inkart ON inkart.art = artikel.art 

        WHERE lev = 1) AS soorten

      AND COUNT(srtc) IN (

                SELECT COUNT(DISTINCT srtc) AS temp

                FROM artikel

                    JOIN inkart ON inkart.art = artikel.art

                WHERE lev = 1

                ) AS aantalsrt

    GROUP BY klant

     ) AS klanten;