Get minimum price with left join / result reduced - mysql

Question

I am coding an online shop. A product has a default price, but since it might have different attributes (color, size, ...), those different attributes might result in different prices as well. At the moment I am trying to produce a mysql query which helps me to find the minimum and maximum possible price of a product. Using LEFT JOIN currently reduces the query's result to only one row and I don't know why.
SELECT
products.id AS id,
categories.name AS category_name,
MIN(product_attributes.price) AS min_price, MAX(product_attributes.price) AS max_price,
products.*
FROM products
LEFT JOIN categories ON category_id=categories.id
LEFT JOIN product_attributes ON products.id=product_attributes.product_id
Is this even the correct approach? I do not know that much about mysql, I just try and try and I am happy if it works. Thanks for help anyway.

Answer 1

You are missing a GROUP BY clause in your current query, but I would recommend using a subquery to get the result:
SELECT
p.id AS id,
c.name AS category_name,
pa.min_price,
pa.max_price,
p.*
FROM products p
LEFT JOIN categories c
ON p.category_id = c.id
LEFT JOIN
(
select MIN(product_attributes.price) min_price,
MAX(product_attributes.price) max_price,
product_id
from product_attributes
group by product_id
) pa
ON p.id=pa.product_id
The main reason why I would suggest using a subquery is because MySQL MySQL uses a EXTENSION TO GROUP BY which allows the behavior of not enforcing the FULL GROUP BY.
This extension in MySQL can cause unexpected values to be returned in the columns in the SELECT list that are not in the GROUP BY clause or in an aggregate function.
From the MySQL Docs:
MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. ... 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 the server chooses.

Answer 2

you need to have GROUP BY clause,
SELECT
products.id AS id,
categories.name AS category_name,
MIN(product_attributes.price) AS min_price,
MAX(product_attributes.price) AS max_price.
products.*
FROM products
LEFT JOIN categories ON category_id=categories.id
LEFT JOIN product_attributes ON products.id=product_attributes.product_id
GROUP BY products.id, categories.name
but be careful when using GROUP BY in mysql, as the statement is perfectly valid if ONLY_FULL_GROUP_BY is disabled by default.
The correct way of doing the query is by using a subquery which separately gets minimum price for each product.
SELECT a.id AS id,
b.name AS category_name,
c.minPrice,
c.maxPrice,
a.*
FROM products a
LEFT JOIN categories b
ON a.category_id = b.id
LEFT JOIN product_attributes c
(
SELECT product_id,
MIN(product_attributes.price) minPrice,
MAX(product_attributes.price) maxPrice
FROM product_attributes
GROUP BY product_id
) d ON a.id = c.product_id