Never Ever

SELECT MAX(`count`), product_id FROM `sales` WHERE product_id='.$productID.

потому что тут только одна колонка должна быть, выкинь product_id

INSERT ON DUPLICATE KEY UPDATE

https://www.mysqltutorial.org/mysql-insert-or-upda...

SET @del := (SELECT CONCAT('DELETE FROM ', GROUP_CONCAT(TABLE_NAME SEPARATOR ' WHERE id=0; DELETE FROM ') , ';' )
  AS statement FROM information_schema.TABLES WHERE TABLE_NAME LIKE '%table_%' );
PREPARE stmt FROM @del;
EXECUTE stmt;