Question

Have a table called user_products that contains a record for each product the user has licensed.

user_id, product_id, (a few other fields like timestamp)

I have 18 products, but want to find all users that have a record for product 3 AND a record for product 4, then delete the record that they have for product 3.

Answer 1

DELETE 
FROM user_products AS up3
WHERE product_id = 3
  AND EXISTS 
      ( SELECT *
        FROM user_products AS up4
        WHERE up4.product_id = 4
          AND up4.user_id = up3.user_id
      )

or with JOIN:

DELETE up3
FROM user_products AS up3
  JOIN user_products AS up4
    ON up4.user_id = up3.user_id
WHERE up3.product_id = 3
  AND up4.product_id = 4