Question

I have a table carts and cartitems, the second one has a foreign key to the first. Now I want to delete all rows from carts that are older than 3 months and have no related cartitems. The following query gives me the correct result:

SELECT * 
FROM  `carts` c
LEFT OUTER JOIN  `cartitems` i ON (  `c`.`id` =  `i`.`cart_id` ) 
WHERE  `c`.`last_updated` < DATE_SUB(NOW() , INTERVAL 3 MONTH) 
GROUP BY  `c`.`id` 
HAVING COUNT(  `i`.`id` ) = 0;

But I can’t figure out how to turn this query into a DELETE.

Also, since there are ~10 million rows in the carts table, I’d be thankful for suggestions on how to improve this query 🙂

Answer 1

You can run the following without LIMIT or with a LIMIT to delete the rows in batches.

DELETE c
FROM  carts c
WHERE c.last_updated < DATE_SUB(NOW() , INTERVAL 3 MONTH)
  AND NOT EXISTS
      ( SELECT *
        FROM cartitems i 
        WHERE c.id = i.cart_id
     ) 
LIMIT 10000 ;                           --- optional