I finally got it to work with some help.

CREATE VIEW qry_Seizoen_Overzicht_Leden AS SELECT Lid_ID, Lid_Achternaam AS Achternaam,

Lid_Tussenvoegsel AS Tussen, Lid_Voornaam AS Voornaam, Lid_Initialen AS Initialen,

Adm_Sei_ID AS Sei_ID,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="BL" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS BL,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="CO" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS CO,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="CS" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS CS,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="JS" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS JS,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="LA" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS LA,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="SC" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS SC,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="SP" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS SP,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="TB" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS TB,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="TR" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS TR,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="WS" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS WS,

CASE WHEN SUM(CASE WHEN Srt_Afkorting="ZT" THEN 1 ELSE 0 END) > 0 THEN "X" ELSE NULL END AS ZT

FROM qry_Overzicht_Leden LEFT JOIN tbl_Administratie ON

tbl_Administratie.Adm_Lid_ID=qry_Overzicht_Leden.Lid_ID LEFT JOIN

tbl_SoortLid ON tbl_SoortLid.Srt_ID = tbl_Administratie.Adm_SoortLid

GROUP BY 1,2,3,4,5,6 ORDER BY Lid_Achternaam, Lid_Voornaam, Adm_Sei_ID