select min value of a field from joins table - mysql

Question

CREATE VIEW products_view
AS
Hi guys ! I've tree tables:
Products
Categories
Prices
A product belongs to one category and may has more prices.
consider this set of data:
Product :
id title featured category_id
1 | bread | yes | 99
2 | milk | yes | 99
3 | honey | yes | 99
Price :
id product_id price quantity
1 | 1 | 99.99 | 10
2 | 1 | 150.00 | 50
3 | 2 | 33.10 | 20
4 | 2 | 10.00 | 11
I need to create a view, a full list of products that for each product select the min price and its own category.
eg.
id title featured cat.name price quantity
1 | bread | yes | food | 99.99 | 10
I tried the following query but in this way I select only the min Price.price value but Price.quantity, for example, came from another row. I should find the min Price.price value and so use the Price.quantity of this row as correct data.
CREATE VIEW products_view
AS
SELECT `Prod`.`id`, `Prod`.`title`, `Prod`.`featured`, `Cat`.`name`, MIN(`Price`.`price`) as price,`Price`.`quantity`
FROM `products` AS `Prod`
LEFT JOIN `prices` AS `Price` ON (`Price`.`product_id` = `Prod`.`id`)
LEFT JOIN `categories` AS `Cat` ON (`Prod`.`category_id` = `Cat`.`id`)
GROUP BY `Prod`.`id`
ORDER BY `Prod`.`id` ASC
My result is:
id title featured cat.name price quantity
1 | bread | yes | food | 99.99 | **50** <-- wrong
Can you help me ? Thx in advance !

Answer 1

As documented under MySQL Extensions to GROUP BY (emphasis added):
In standard SQL, a query that includes a GROUP BY clause cannot refer to nonaggregated columns in the select list that are not named in the GROUP BY clause. For example, this query is illegal in standard SQL because the name column in the select list does not appear in the GROUP BY:
SELECT o.custid, c.name, MAX(o.payment)
FROM orders AS o, customers AS c
WHERE o.custid = c.custid
GROUP BY o.custid;
For the query to be legal, the name column must be omitted from the select list or named in the GROUP BY clause.
MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. This means that the preceding query is legal in MySQL. You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values within each group the server chooses.
What you are looking for is the group-wise minimum, which can be obtained by joining the grouped results back to the table:
SELECT Prod.id, Prod.title, Prod.featured, Cat.name, Price.price, Price.quantity
FROM products AS Prod
LEFT JOIN categories AS Cat ON Prod.category_id = Cat.id
LEFT JOIN (
prices AS Price NATURAL JOIN (
SELECT product_id, MIN(price) AS price
FROM prices
GROUP BY product_id
) t
) ON Price.product_id = Prod.id
ORDER BY Prod.id