Assuming I understand what you're asking, this is really a SQL question rather than a Perl question. That caveat aside, it looks like you need to use an outer join, e.g. something like this:

SELECT items.it_name, user_item_history.ID 
FROM items
LEFT JOIN user_item_history 
	ON (user_item_history.it_ID = items.it_ID)

This will give you all items, and include the ID of any associated history items. Any item that has no history will still return a row with a NULL user_item_history.ID.

-- grummerX