you'd have to compare the loottable assigned to the mob to any other mob with the assigned loot table.
Someone who knows more about SQL than I would have to help you out, though.
Theorycrafting:
Compare loottable assigned to any mob with all other mobs who have the same loottable
if any of them are lower than X, don't add it, otherwise, add it.
maybe something like this for starters (assuming these are the right table and column names)
Code:
select * from lootdrop INNER JOIN npc_types on lootdrop.id = npc_types.lootdrop_id;
select lootdrop .*,npc_types .* from lootdrop ,npc_types where lootdrop.id = npc_types.lootdrop_id;