MySQL check if value already exists between two tables

Can you be more specific on the fields?

Does your sub_items table have itemsId?

If yes, I think you can join those two tables and search for the new item?

Something like this:

SELECT I.item_id AS item_items_id, SI.item_id AS subitem_items_id FROM items I
INNER JOIN sub_items SI ON I.item_id = SI.item_id AND I.asset_number = SI.asset_number
WHERE I.item_id = -- if your new item has an item_id you can search it

If the new item doesn't have an ID, then maybe you can search by using a item name if there is a column?