SQL query to get SUM from more dependent tables - mysql

Question

I have the following MySQL DB structure:
table sales_order - id, name, ...
id | name
------------------
1 | Order Test
table sales_order_item - id, order_id, name, amount_dispatched ...
id | order_id | name | amount_dispatched
------------------------------------------
1 | 1 | Item 1 | 5
2 | 1 | Item 2 | 10
table sales_order_item_invoice - id, item_id, amount, ...
id | item_id | amount
---------------------
1 | 1 | 3
2 | 2 | 5
3 | 2 | 5
These three tables are in chain via the foreign keys. Table "invoice" can have more rows for one row in "item". Table "item" can have more rows for one row in "order".
Now, I need to create SQL query that returns all rows from table sales_order and appends there some data from the other tables - amount_dispatched and amount_invoiced:
dispatched = sum of all order's items' amount_dispatched
invoiced = sum of all invoices' amount (or 0 if no invoice exists)
Such query seems to be straightforward:
SELECT
`sales_order`.*,
SUM(`sales_order_item`.`amount_dispatched`) AS dispatched,
SUM(`sales_order_item_invoice`.`amount`) AS invoiced,
FROM `sales_order`
LEFT JOIN `sales_order_item` ON `sales_order`.`id` = `sales_order_item`.`order_id`
LEFT JOIN `sales_order_item_invoice` ON `sales_order_item`.`id` =`sales_order_item_invoice`.`item_id`
GROUP BY `sales_order`.`id`
The result contains all orders - ok
The result contains sum of invoices amount - ok
The result of "amount_dispatched" is invalid - if the item has more rows in item_invoice, the item's amount is summed several times, so for the example above, I get:
id | name | dispatched | invoiced
---------------------------------------
1 | Order Test | 25 | 13
Amount_dispatched is 25, but I would expect it to be 15.
Any idea how to correct my SQL query?
Thank you.

Answer 1

Firstly, use subquery do aggregation for invoice amount in sales_order_item_invoice, then left join.
SELECT
`sales_order`.*,
SUM(`sales_order_item`.`amount_dispatched`) AS dispatched,
SUM(t.`amount`) AS invoiced
FROM `sales_order`
LEFT JOIN `sales_order_item` ON `sales_order`.`id` = `sales_order_item`.`order_id`
LEFT JOIN (
SELECT item_id, SUM(amount) AS amount
FROM `sales_order_item_invoice`
GROUP BY item_id
) t ON `sales_order_item`.`id` = t.`item_id`
GROUP BY `sales_order`.`id`