Question

My problem is I want to delete records from two tables, deleting the records that are a result of the following query:

SELECT AVG(pr.rating) AS rating_average
FROM products p 
INNER JOIN product_ratings pr 
ON pr.product_id = p.product_id  
GROUP BY p.product_id 
HAVING AVG(pr.rating) < 3

The above query shows average ratings of a product that are less than 3, I want to delete products and their associated ratings of all the results from the above query.

I looked at DELETE FROM product_ratings, products WHERE EXISTS (//above query), but this didn’t work, I’ve been trying various DELETE statements to no avail.

I have read the following, and still cannot find a solution: SQL: DELETE Statement & SQL: EXISTS Condition.

The tables are products and product_ratings, with the following structure:

products
--------
product_id [PK] | link | ...

product_ratings
---------------
rating_id [PK] | rating | product_id

Appreciate any help, as well as links to reference material to better understand how it’s done.

EDIT: Apologies for not stating what RDBMS I’m using, It’s MySQL

EDIT2: A bit confused now, @Martin’s example doesn’t use a temp table like the other answers, I assume this is because of my vague question not stating with RDBMS I was using?

Answer 1

You have now added the MySQL tag. In that case this might do the job for you.

DELETE products,
       product_ratings
FROM   products,
       product_ratings
WHERE  product_ratings.product_id = products.product_id
       AND product_ratings.product_id IN 
 (SELECT product_id
        FROM   (SELECT p.product_id
                FROM   products p
                       LEFT JOIN product_ratings pr
                         ON pr.product_id = p.product_id
                GROUP  BY p.product_id
                HAVING COALESCE(AVG(pr.rating), 0) < 3) T) 

MySQL does support a multiple table DELETE syntax. The derived table is to get around the issue where it doesn’t allow mutating (update or delete target) tables to be referenced in a sub query (it materializes the result into a temporary table).