Dies scheint eine korrekte Ergebnismenge zu erzeugen:
SELECT col1.id, col1.holder, col2.borrowMax, count(lend.borrowedId) as `count`
FROM collection_db col1
INNER JOIN collection_db col2
ON col1.holder = col2.id
INNER JOIN lendings lend
ON col1.holder = lend.holder
WHERE col1.id = $id
AND col2.category = 10
AND lend.memId = $medId