Question

I have two MySQL tables:

• product has id, title, price and some other product attributes
• tag has *product_id* and *tag_name* fields

each product row can have one ore more rows in table tag joined by *product_id* field.
Question: how do I select products by *tag_name* and other attributes?
Example: I need to select all books cheaper than $10 and tagged ‘foreign’ and ‘sci-fi’

I know how to select products with one tag (join product and tag by *product_id* and set where clause)
I know how to select products with more than one tag using a stored procedure, however queries take very long time to run.
Is there a way to avoid stored procedure and still get the result?

Update: StackOverflow does something like this when you click “Related Tags” on the right hand side bar. Now, I don’t know what architechture their DB has, but the result is fairly quick selection of all questions tagged with multiple tags.

Answer 1

This will return any product with price less than 10, and with tag foreign or sci-fi

How about something like

SELECT  p.*
FROM    product p
WHERE   p.price < 10
AND     EXISTS  (
                    SELECT  1
                    FROM    tag t
                    WHERE   p.id = t.product_id
                    AND     t.tag_name IN ('foreign','sci-fi')
                )

This will ensure that it has both tags, assuming that tags has primary key product_id and tag_name

SELECT  p.*
FROM    product p
WHERE   p.price < 10
AND     EXISTS  (
                    SELECT  1
                    FROM    tag t
                    WHERE   p.id = t.product_id
                    AND     t.tag_name IN ('foreign','sci-fi')
                    HAVING COUNT(1) = 2
                )