Résolu !
SELECT
d.id,
c.cmc,
(CAST(SUM(m.count) AS FLOAT) /
(SELECT
CAST(SUM(m1.count) AS FLOAT)
FROM deckmembers AS m1
JOIN cards AS c1 ON c1.id=m1.card
WHERE NOT m1.sideboard
AND c1.fulltype NOT LIKE '%Land%'
AND m1.deck=d.id)
) * 100 AS perc
FROM deckmembers AS m
JOIN decks AS d ON d.id=m.deck
JOIN cards AS c ON c.id=m.card
WHERE NOT m.sideboard
AND c.fulltype NOT LIKE '%Land%'
GROUP BY d.id, c.cmc;
J'ai également posté une solution à la version simplifiée du problème ici .