MySQL正确的一对多关系包括和不包括右表的联接方法

Question

I've been trying to solve this MySQL issue...half of the problem is wording my question, I apologize if this has been solved before, I did a lot of searching but wasn't able to find a solution.

I have two tables, called orders and order_items Each order can contain many order items.

I need to do a search that returns orders that contain only one order item and not another, then vice versa.

To illustrate this point:

Table orders

order_id
1
2
3

Table order_items

order_item_id | order_item_type | order_id
1             | A               | 1
2             | A               | 2
3             | B               | 2
4             | B               | 3

What I need to do is to get orders that include only item A, only item B, and both.

As you can see Only Item A should return order 1, only item B should return order 3, and both should return order 2.

Despite all my best attempts I cannot get anything working. Here is the current attempt at returning orders with ONLY item A

SELECT *
    FROM orders
    LEFT OUTER JOIN order_items
    ON order_items.order_id = orders.order_id
    WHERE order_items.order_item_type IN ('A')
    AND order_items.order_item_type NOT IN ('B')
    GROUP BY orders.order_id

As an experienced person can probably guess looking at my attempt, this returns both order 1 and order 2. I think this could probably be done with a subquery, but I believe it can be done with a proper join...just not sure how to do it. Thank you!

Answer 1

when selected single order_item:

SELECT i.* FROM order_items i
WHERE i.order_item_type = 'A' AND
  (SELECT COUNT(order_id) FROM order_items WHERE order_id = i.order_id) = 1

selected two order_item:

SELECT i.* FROM order_items i
WHERE i.order_item_type IN ('A', 'B') AND
  (SELECT COUNT(order_id) FROM order_items WHERE order_id = i.order_id) = 2

and join orders table as you like.