Things to do,
- use
INSERT...INTO SELECT
statement - use
LEFT JOIN
instead ofNOT IN
- don't forget to add index on this columns:
products.ID
anddownloads.product_id
for faster performance
query,
INSERT INTO downloads (product_id, filename)
SELECT a.id AS product_id, 'default.zip' AS filename
FROM products a
LEFT JOIN downloads b
ON a.ID = b.product_id
WHERE b.product_id IS NULL