MySQL Retrieve Lowest Value in Multi-table Query - mysql

Question

My goal is to retrieve the recorded purchase price for an item on an accepted purchase order.
Purchase_Orders table contains metadata for the order, such as the order number and its status (e.g., 1 for accepted, 0 for declined).
Purchase_Ord_Contents table contains contents records, which are linked via foreign key to the parent purchase order on a shared index order_number)
For example: I have two orders in my database, one has been accepted and the other has been declined. The data is represented as follows:
=========================================
PURCHASE_ORDERS TABLE
=========================================
id | order_number | order_status
-----------------------------------------
1 PO_100 0
2 PO_101 1
3 PO_102 1
===================================================
PURCHASE_ORD_CONTENTS TABLE
===================================================
id | order_number | purchase_price | sku
---------------------------------------------------
1 PO_100 1.50 APPLE
2 PO_100 1.50 ORANGE
3 PO_101 2.00 APPLE
4 PO_101 2.00 ORANGE
5 PO_102 1.75 BANANA
The query should return rows 3, 4 and 5, since PO_101 was accepted, whereas PO_100 was declined and row 5 is not only the only record for the given SKU, it was also on an accepted order. I've tried a few different approaches, but I always seem to end up either leaving out parts that were on an unaccepted Purchase Order, or retrieving the wrong order_number for the lowest purchase_price.
Here is what I have thus far (not working properly)
SELECT a.*
FROM purchase_ord_contents AS a
JOIN (SELECT sku,
MIN(purchase_price) AS min_price
FROM purchase_ord_contents
GROUP BY sku) AS b
ON ( a.sku = b.sku
AND a.purchase_price = b.min_price )
WHERE a.order_number
IN (
SELECT order_number
FROM purchase_orders
WHERE order_status != 0
)
This query successfully returns the records from the purchase_ord_contents table, however it omits records of the lowest purchase_price that were on a Purchase Order with an order_status of 0.
Any guidance would be greatly appreciated, I am not very well versed in "advanced" SQL queries as you have probably determined by now. Thank you for your time and please do not hesitate to ask if I should provide any further information.

Answer 1

This could be what you are looking for:
SELECT sku, purchase_price, order_number
FROM (
SELECT MIN(purchase_price) AS purchase_price, sku
FROM purchase_ord_contents
JOIN purchase_orders USING (order_number)
WHERE purchase_orders.order_status = 1
GROUP BY sku
) AS min_sku_price -- this is the lowest sale price for each SKU
JOIN purchase_ord_contents USING (sku, purchase_price) -- gets all orders having sold a SKU at its lowest price
JOIN purchase_orders USING (order_number)
WHERE purchase_orders.order_status = 1
Notice this will return several rows for one given SKU if the lowest price for this SKU was offered in several orders.

Answer 2

If I understand correctly I think you want this:
SELECT po.order_number, poc.sku, min(poc.purchase_price)
FROM purchase_orders AS po
JOIN purchase_ord_contents AS poc ON poc.order_number = po.order_number
WHERE po.order_status != 0
GROUP by po.order_number, poc.sku
order by po.order_number, poc.sku