MySql sum DISTINCT after join - mysql

Question

I have 2 table that join together ( Orders and Order_item)
When I use join I get duplicates records then I eliminate them with DISTINCT, but when I want to get sum the the shipping_price DISTINCT not works because it just eliminate the same price value and I all my prices are same I get 1 not sum !
If I don't use DISTINCT , I get rows per each items in order
SELECT sum(DISTINCT shipping_price) FROM `product_order`
INNER JOIN `product_address`
ON `product_order`.`address_id` = `product_address`.`id`
INNER JOIN `product_item`
ON `product_order`.`id` = `product_item`.`order_id`
WHERE (`status`<>2)
AND (`company_id`=1968)
AND DATE(date)='2021-08-31'
ORDER BY `product_order`.`id` DESC
how to get sum of shipping price correctly ..
data in order_table is
id shipping_price status
100
200
200
100
sum = 600 , but how to get it, if I did't DISTINCT, I get more than one row per order_items row counts that join tho order..

Answer 1

Use a subquery to get your shipping prices. Something like this:
SELECT product_order.id,
SUM(product_item.price * product_item.quantity) shipping_price
FROM product_order
JOIN product_item ON product_order.id = product_item.order_id
GROUP BY product_order.id
The trick here is to get a subquery that delivers exactly one row per order, with the summed-up shipping priced in it. Do your SUM() ... GROUP BY ...` in the subquery. That way you'll avoid any duplication of items.
TEST THIS before you proceed to make sure it works: giving each order id and its shipping price.
Then use it as if it were a table, JOINing it to the rest.
SELECT total.shipping_price,
product_order.id,
product_address.*
FROM product_order
JOIN product_address
ON product_order.id = product_address.order_id
JOIN (
SELECT product_order.id,
SUM(product_item.price * product_item.quantity) shipping_price
FROM product_order
JOIN product_item ON product_order.id = product_item.order_id
GROUP BY product_order.id
) total ON product_order.id = total.id
ORDER BY product_order.id = total.id

Answer 2

Use a subquery instead of a join.
SELECT (
select sum(shipping_price)
FROM product_item
WHERE po.`id` = `product_item`.`order_id`
) as shipping_price
FROM `product_order` po
INNER JOIN `product_address`
ON `product_order`.`address_id` = `product_address`.`id`
WHERE (`status`<>2)
AND (`company_id`=1968)
AND DATE(date)='2021-08-31'
ORDER BY `product_order`.`id` DESC