select sum qty from 2 tables - mysql

Question

In MySQL I have 4 tables:
- product(id)
- order(id)
- order_detail_1(id, product_id, order_id, qty)
- order_detail_2(id, product_id, order_id, qty)
I want to get the sum of the quantity of products sold from the 2 tables (order_detail_1, order_detail_2) grouping them by product
produt can existe in order_detail_1 and not in order_detail_2 and vice versa
i tested this query and it worked but I want a simpler query without the union and the subquery.
select tmp.product_id ,sum(tmp.qty) from
(
(
select order_detail_1.product_id ,sum(order_detail_1.qty)
from order_detail_1
inner join order on order_detail_1.id_order = order.id
where order_detail_1.product_id is not null
group by order_detail_1.product_id
)
union all
(
select order_detail_2.product_id ,sum(order_detail_2.qty)
from order_detail_2
inner join order on order_detail_2.id_order = order.id
where order_detail_2.product_id is not null
group by order_detail_2.product_id
)
) tmp
group by tmp.product_id

Answer 1

It looks like you're not using order table other then checking if it exists, so you can use EXISTS()
SELECT p.product_id,sum(p.qty) as qty
FROM (SELECT product_id,qty,id_order FROM order_detail_1
WHERE product_id IS NOT NULL
UNION ALL
SELECT product_id,qty,id_order FROM order_detail_2
WHERE product_id IS NOT NULL) p
WHERE EXISTS(SELECT 1 FROM order o
WHERE o.id = p.id_order)
GROUP BY p.product_id

Answer 2

If a product is in only one table, you can use left join:
select p.id, (coalesce(sum(od1.qty), 0) + coalesce(sum(od2.qty, 0))) as qty
from product p left join
order_detail_1 od1
on od1.product_id = p.id left join
order_detail_2 od2
on od2.product_id = p.id
group by p.id;
This formulation depends on the fact that the two tables are exclusion -- a product is in only one table.
EDIT:
If products can exist in both tables, then you need to aggregate them first:
select p.id, (coalesce(od1.qty, 0) + coalesce(od2.qty, 0)) as qty
from product p left join
(select product_id, sum(qty) as qty
from order_detail_1 od1
group by product_id
) od1
on od1.product_id = p.id left join
(select product_id, sum(qty) as qty
from order_detail_2 od2
group by product_id
) od2
on od2.product_id = p.id;