[Malleys]

Может так... (и может вместо tb1.id в tb2 надо tb2.tb1_id)

SELECT
    tb1.id, tb1.count, SUM(tb2.count) as used
FROM
    tb1
LEFT JOIN
    tb2
ON
    tb2.tb1_id = tb1.id
GROUP BY
    id, count
HAVING
    count > used
;