Question

I have two tables in MySQL 5.1.38.

products
+----+------------+-------+------------+
| id | name       | price | department |
+----+------------+-------+------------+
|  1 | Fire Truck | 15.00 | Toys       |
|  2 | Bike       | 75.00 | Toys       |
|  3 | T-Shirt    | 18.00 | Clothes    |
|  4 | Skirt      | 18.00 | Clothes    |
|  5 | Pants      | 22.00 | Clothes    |
+----+------------+-------+------------+

ratings
+------------+--------+
| product_id | rating |
+------------+--------+
|          1 |      5 |
|          2 |      5 |
|          2 |      3 |
|          2 |      5 |
|          3 |      5 |
|          4 |      5 |
|          5 |      4 |
+------------+--------+

My goal is to get the total price of all products which have a 5 star rating in each department. Something like this.

+------------+-------------+
| department | total_price |
+------------+-------------+
| Clothes    | 36.00       |  /* T-Shirt and Skirt */
| Toys       | 90.00       |  /* Fire Truck and Bike */
+------------+-------------+

I would like to do this without a subquery if I can. At first I tried a join with a sum().

select department, sum(price) from products
join ratings on product_id=products.id
where rating=5 group by department;
+------------+------------+
| department | sum(price) |
+------------+------------+
| Clothes    |      36.00 |
| Toys       |     165.00 |
+------------+------------+

As you can see the price for the Toys department is incorrect because there are two 5 star ratings for the Bike and therefore counting that price twice due to the join.

I then tried adding distinct to the sum.

select department, sum(distinct price) from products
join ratings on product_id=products.id where rating=5
group by department;
+------------+---------------------+
| department | sum(distinct price) |
+------------+---------------------+
| Clothes    |               18.00 |
| Toys       |               90.00 |
+------------+---------------------+

But then the clothes department is off because two products share the same price.

Currently my work-around involves taking something unique about the product (the id) and using that to make the price unique.

select department, sum(distinct price + id * 100000) - sum(id * 100000) as total_price
from products join ratings on product_id=products.id
where rating=5 group by department;
+------------+-------------+
| department | total_price |
+------------+-------------+
| Clothes    |       36.00 |
| Toys       |       90.00 |
+------------+-------------+

But this feels like such a silly hack. Is there a better way to do this without a subquery? Thanks!

Answer 1

Use:

  SELECT p.department,
         SUM(p.price) AS total_price
    FROM PRODUCTS p
    JOIN (SELECT DISTINCT 
                 r.product_id,
                 r.rating
            FROM RATINGS r) x ON x.product_id = p.id
                             AND x.rating = 5
GROUP BY p.department

Technically, this does not use a subquery – it uses a derived table/inline view.