Question

I have a table called order which contains columns id, user_id, price and item_id. Item prices aren’t fixed and I would like to select each item’s most expensive order. I want to select user_id, item_id and price in the same query. I tried the following query but it doesn’t return the correct result set.

SELECT user_id, item_id, MAX(price)
FROM order
GROUP BY item_id

Some of the rows returned by this query have the wrong user_id. However, all rows in the result set show each item’s correct highest price.

Answer 1

You may want to use a derived table, as follows:

SELECT    o1.item_id, o1.max_price, o2.user_id user_of_max_price
FROM      (
             SELECT item_id, MAX(price) max_price
             FROM `order`
             GROUP BY item_id
          ) o1
JOIN      `order` o2 ON (o2.price = o1.max_price AND o2.item_id = o1.item_id)
GROUP BY  o1.item_id;

Test case:

CREATE TABLE `order` (user_id int, item_id int, price decimal(5,2));

INSERT INTO `order` VALUES (1, 1, 10);
INSERT INTO `order` VALUES (1, 2, 15);
INSERT INTO `order` VALUES (1, 3, 8);
INSERT INTO `order` VALUES (2, 1, 20);
INSERT INTO `order` VALUES (2, 2, 6);
INSERT INTO `order` VALUES (2, 3, 15);
INSERT INTO `order` VALUES (3, 1, 18);
INSERT INTO `order` VALUES (3, 2, 13);
INSERT INTO `order` VALUES (3, 3, 10);

Result:

+---------+-----------+-------------------+
| item_id | max_price | user_of_max_price |
+---------+-----------+-------------------+
|       1 |     20.00 |                 2 |
|       2 |     15.00 |                 1 |
|       3 |     15.00 |                 2 |
+---------+-----------+-------------------+
3 rows in set (0.00 sec)