3 Bottle oil for Outboard motor

1055 Expression #1 of ORDER BY clause is not in GROUP BY clause and contains nonaggregated column 'yamaha_motor.o.date_purchased' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
in:
[select p.products_id, p.products_image from orders_products opa, orders_products opb, orders o, products p where opa.products_id = '639' and opa.orders_id = opb.orders_id and opb.products_id != '639' and opb.products_id = p.products_id and opb.orders_id = o.orders_id and p.products_status = 1 group by p.products_id order by o.date_purchased desc limit 6]