Question

I have a table containing perfectly defined items and a second table with potentially vague/greedy orders as NULL would require all available values for this parameter.

items
+-----------------------+
| item_id | color | size |
|---------+-------+------|
|    1    |  blue |    8 |
|    2    |   red |    6 |
|    3    | green |    7 |
|    4    | black |    6 |
+------------------------+

orders
+-------------------------+
| order_id | color | size |
|----------+-------+------|
|     1    |   red |    6 |
|     2    | green |    8 |
|     3    |  NULL |    6 |
|     4    |  blue | NULL |
|     5    |  NULL | NULL |
+-------------------------+

Is there an efficient way to generate a complete list of items needed to fill all orders?

+--------------------+
| order_id | item_id |
|----------+---------|
|     1    |    2    |
|     3    |    2    |
|     3    |    4    |
|     4    |    1    |
|     5    |    1    |
|     5    |    2    |
|     5    |    3    |
|     5    |    4    |
+--------------------+

It seems to me like an INNER JOIN should be able to do this, but something like this obviously doesn’t consider the possibility of NULL values as greedy wildcards in the orders table:

SELECT order_id, item_id
FROM orders
INNER JOIN items ON orders.color = items.color AND orders.size = items.size

Any ideas?

Answer 1

Try the following:

SELECT order_id, item_id
FROM orders
INNER JOIN items ON (orders.color IS NULL OR orders.color = items.color)
    AND (orders.size IS NULL OR orders.size = items.size)

Let me know if that helps, or if I misunderstood the question.