Getting associated data from max column with multiple tables in MySQL - mysql

Question

I have two tables, Orders and OrderDetails. I would like to create an SQL query that retrieves the OrderID, Unit Cost and Process Date for orders with the latest processed T-Shirts for each user.
I have created a very simplified version of my tables in this SQL fiddle.
From my google searches, most of which ended up here on SO, I have managed to figure out SQL to get the associated unit cost for the latest processed order of T-shirts per order, but I'm not sure how to further condense it from here to be for each user and still maintain the correct unit cost and process dates. I know I can't use MAX and GROUP BY and I can't work out how to do a LEFT OUTER JOIN (like how I did the first part in the SQL fiddle) when the UserID and ProcessDate reside in different tables.
For the dataset in the SQL fiddle, the output I would be looking for would be
| UserID | OrderID | UnitCost | ProcessDate |
|-------- |--------- |---------- |------------- |
| 1 | 2 | 3 | 2018-03-01 |
| 2 | 3 | 1 | 2018-04-01 |
| 3 | 5 | 3 | 2018-06-01 |

Answer 1

Above query output is perfectly fine just the case is correlated subquery gets executed everytime for each row which is quite a overhead and in case of large data there will be performance issue.
I have changed the subquery into join which may be helpful.
SELECT O.UserID
, O.OrderID
, OD.UnitCost
, OD.ProcessDate
FROM Orders O
left
JOIN OrderDetails OD
ON O.OrderID = OD.OrderID
left
join
( SELECT o2.userid
, max(od2.processdate) newdate
FROM Orders o2
JOIN OrderDetails od2
ON o2.OrderId = od2.OrderId
WHERE od2.Product = 'T-Shirts'
group
by o2.userid
) temp
on O.userid = temp.userid
WHERE OD.Product = 'T-Shirts'
and OD.processdate = temp.newdate
ORDER
BY OD.OrderDetailID;
Hope it helps.

Answer 2

You can select the "latest" order for each user using a correlated subquery:
SELECT O.UserID, O.OrderID, OD.UnitCost, OD.ProcessDate
FROM OrderDetails OD LEFT JOIN
Orders O
ON O.OrderID = OD.OrderID
WHERE OD.Product = 'T-Shirts' AND
o.OrderID = (SELECT MAX(o2.OrderId)
FROM Orders o2
WHERE o2.UserID = o.UserID
)
ORDER BY OD.OrderDetailID;
This assumes that the latest order is the one with the largest OrderId.
EDIT:
Actually, the above happens to work on your data, because the largest order id for each user has t-shirts. However, you explicitly say that you want the latest order with t-shirts, so that requires a slight modification:
SELECT O.UserID, O.OrderID, OD.UnitCost, OD.ProcessDate
FROM Orders O LEFT JOIN
OrderDetails OD
ON O.OrderID = OD.OrderID
WHERE OD.Product = 'T-Shirts' AND
o.OrderID = (SELECT MAX(o2.OrderId)
FROM Orders o2 JOIN
OrderDetails od2
ON o2.OrderId = od2.OrderId
WHERE od2.Product = 'T-Shirts' AND
o2.UserID = o.UserID
)
ORDER BY OD.OrderDetailID;
Or, if you want to go by process date:
SELECT O.UserID, O.OrderID, OD.UnitCost, OD.ProcessDate
FROM Orders O LEFT JOIN
OrderDetails OD
ON O.OrderID = OD.OrderID
WHERE OD.Product = 'T-Shirts' AND
od.ProcessDate = (SELECT od2.ProcessDate
FROM Orders o2 JOIN
OrderDetails od2
ON o2.OrderId = od2.OrderId
WHERE od2.Product = 'T-Shirts' AND
o2.UserID = o.UserID
ORDER BY od2.ProcessDate DESC
LIMIT 1
)
ORDER BY OD.OrderDetailID;