How to get multiple columns on subquery or group by - mysql

Question

I have two tables on MySql, the first contains an ID and the name of some products. I have to get the cheapest combination of brand/market for each product. So, I've inserted some itens into both tables:
UPDATE: Inserted new product (bed) with no 'Product_Brand_Market' to test LEFT JOIN.
UPDATE: Changed some product prices for better testing.
CREATE TABLE Product(
id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(20) NOT NULL);
CREATE TABLE Product_Brand_Market(
product INT UNSIGNED,
market INT UNSIGNED, /*this will be a FOREIGN KEY*/
brand INT UNSIGNED, /*this will be a FOREIGN KEY*/
price DECIMAL(10,2) UNSIGNED NOT NULL,
PRIMARY KEY(product, market, brand),
CONSTRAINT FOREIGN KEY (product) REFERENCES Product(id));
INSERT INTO Product
(name) VALUES
('Chair'), /*will get id=1*/
('Table'), /*will get id=2*/
('Bed'); /*will get id=3*/
INSERT INTO Product_Brand_Market
(product, market, brand, price) VALUES
(1, 1, 1, 8.00), /*cheapest chair (brand=1, market=1)*/
(1, 1, 2, 8.50),
(1, 2, 1, 9.00),
(1, 2, 2, 9.50),
(2, 1, 1, 11.50),
(2, 1, 2, 11.00),
(2, 2, 1, 10.50),
(2, 2, 2, 10.00); /*cheapest table (brand=2, market=2)*/
/*no entries for bed, must return null*/
And tried the following code to get the desired values:
UPDATE: Changed INNER JOIN for LEFT JOIN.
SELECT p.id product, MIN(pbm.price) price, pbm.brand, pbm.market
FROM Product p
LEFT JOIN Product_Brand_Market pbm
ON p.id = pbm.product
GROUP BY p.id;
The returned price is OK, but I'm getting the wrong keys:
| product | price | brand | market |
|---------|-------|-------|--------|
| 1 | 8 | 1 | 1 |
| 2 | 10 | 1 | 1 |
| 3 | null | null | null |
So the only way I could think to solve it is with subqueries, but I had to use two subqueries to get both brand and market:
SELECT
p.id product,
(
SELECT pbm.brand
FROM Product_Brand_Market pbm
WHERE p.id = pbm.product
ORDER BY pbm.price
LIMIT 1
) as brand,
(
SELECT pbm.market
FROM Product_Brand_Market pbm
WHERE p.id = pbm.product
ORDER BY pbm.price
LIMIT 1
) as market
FROM Product p;
It returns the desired table:
| product | brand | market |
|---------|-------|--------|
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 3 | null | null |
But I want to know if I really should use these two similar subqueries or there is a better way to do that on MySql, any ideas?

Answer 1

Use a correlated subquery with LIMIT 1 in the WHERE clause:
SELECT product, brand, market
FROM Product_Brand_Market pbm
WHERE (pbm.brand, pbm.market) = (
SELECT pbm1.brand, pbm1.market
FROM Product_Brand_Market pbm1
WHERE pbm1.product = pbm.product
ORDER BY pbm1.price ASC
LIMIT 1
)
This will return only one row per product, even if there are two or many of them with the same lowest price.
Demo: http://rextester.com/UIC44628
Update:
To get all products even if they have no entries in the Product_Brand_Market table, you will need a LEFT JOIN. Note that the condition should be moved to the ON clause.
SELECT p.id as product, pbm.brand, pbm.market
FROM Product p
LEFT JOIN Product_Brand_Market pbm
ON pbm.product = p.id
AND (pbm.brand, pbm.market) = (
SELECT pbm1.brand, pbm1.market
FROM Product_Brand_Market pbm1
WHERE pbm1.product = pbm.product
ORDER BY pbm1.price ASC
LIMIT 1
);
Demo: http://rextester.com/MGXN36725
The follwing query might make a better use of your PK for the JOIN:
SELECT p.id as product, pbm.brand, pbm.market
FROM Product p
LEFT JOIN Product_Brand_Market pbm
ON (pbm.product, pbm.market, pbm.brand) = (
SELECT pbm1.product, pbm1.market, pbm1.brand
FROM Product_Brand_Market pbm1
WHERE pbm1.product = p.id
ORDER BY pbm1.price ASC
LIMIT 1
);
An index on Product_Brand_Market(product, price) should also help to improve the performance of the subquery.