How to select a distinct column value matching multiple criterea - mysql

Question

I have a table containing attributes with the following structure:
id: bigint unsigned autoincrement
product_id: bigint foreign key
attribute_id: bigint foreign key
value: varchar(100)
I can query one criteria in the following fashion:
SELECT DISTINCT product_id FROM product_attributes WHERE attribute_id = ? AND value = ?
However I need to be able to find products that match multiple such criteria and would like to avoid multiple database queries for performance reasons. Simply adding more criteria with AND won't work since they will involve the same columns so for example:
SELECT DISTINCT product_id FROM product_attributes WHERE attribute_id = 1 AND value = 'Blue'
INTERSECT
SELECT DISTINCT product_id FROM product_attributes WHERE attribute_id = 2 AND value = '36'
INTERSECT
SELECT DISTINCT product_id FROM product_attributes WHERE attribute_id = 3 AND value = 'slim'
I have read about the INTERSECT statement which seems like it might work but I've read that MySQL doesn't support it, a search through MySQL 8 documentation produced no relevant result and the query above which I assume is correct produces an error on MySQL.
I've also read that something similar could be achieved with an inner join, but all the examples I've found involve multiple tables. There might also be an even better or simpler way to write the query that hasn't occurred to me. Or perhaps it's actually better to just send multiple queries and calculate the intersection outside of MySQL (though I would be very surprised) I appreciate greatly any help from anyone who has done something similar in the past.

Answer 1

You need to use aggregation to count the number of matching rows to the set of conditions and assert that it is equal to the number of conditions:
SELECT product_id
FROM product_attributes
WHERE (attribute_id, value) IN ((1, 'Blue'), (2, '36'), (3, 'slim'))
GROUP BY product_id
HAVING COUNT(*) = 3

Answer 2

This is the key / value store problem.
It's a slight pain in the neck to do what you want. Use JOIN operations to pivot the values into a row. Like this.
SELECT p.product_id,
color.value AS color,
size.value AS size,
cut.value AS cut
FROM ( SELECT DISTINCT product_id FROM product_attributes ) p
LEFT JOIN product_attributes color ON color.product_id = p.product_id
AND color.attribute_id = 1
LEFT JOIN product_attributes size ON size.product_id = p.product_id
AND size.attribute_id = 2
LEFT JOIN product_attributes cut ON cut.product_id = p.product_id
AND cut.attribute_id = 3
This generates a resultset with one row per product/color/size/cut combination
Then you can filter that resultset like this
SELECT *
FROM (
SELECT p.product_id,
color.value AS color,
size.value AS size,
cut.value AS cut
FROM ( SELECT DISTINCT product_id FROM product_attributes ) p
LEFT JOIN product_attributes color ON color.product_id = p.product_id
AND color.attribute_id = 1
LEFT JOIN product_attributes size ON size.product_id = p.product_id
AND size.attribute_id = 2
LEFT JOIN product_attributes cut ON cut.product_id = p.product_id
AND cut.attribute_id = 3
) combinations
WHERE color='Blue' AND size='36' AND cut='slim'
MySQL's query planner is smart enough that this doesn't run as slowly as you might guess, given the proper indexes.
The FROM clause generates a comprehensive list of product ids, from your product_attributes table to join to the specific attributes. If you have some other table for products, use that instead of the SELECT DISTINCT....