The previous code over-counted as the use of LEFT JOINs meant candidate
group rows with the incorrect container_id were still included in the
results. Using an IN clause with a subquery allows us to remove those
rows, though it's a bit hacky. (Really I want to be able to use a JOIN
on the right side of a LEFT JOIN to restrict the rows that appear in the
results of the LEFT JOIN, but that doesn't seem to be possible.)
Signed-off-by: Graham <gpe@openrs2.org>