$sql_del = "
DELETE
sales_centers
FROM sales_centers
LEFT JOIN orders USING(id_center)
WHERE sales_centers.id_center IN(5, 7)
AND orders.id_order IS NULL
GROUP BY sales_centers.id_center";
ругается:
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'GROUP BY sales_centers.id_center' at line 7