SUM values from two tables with GROUP BY and WHERE - postgresql

Question

I have two tables below named sent_table and received_table. I am attempting to mash them together in a query to achieve output_table. All my attempts so far result in a huge amount of duplicates and totally bogus sum values.
I am assuming I would need to use GROUP BY and WHERE to achieve this goal. I want to be able to filter based on the users name.
sent_table
+----+------+-------+----------+
| id | name | value | order_id |
+----+------+-------+----------+
| 1 | dave | 100 | 1 |
| 2 | dave | 200 | 1 |
| 3 | dave | 300 | 2 |
+----+------+-------+----------+
received_table
+----+------+-------+----------+
| id | name | value | order_id |
+----+------+-------+----------+
| 1 | dave | 400 | 1 |
| 2 | dave | 500 | 2 |
| 3 | dave | 600 | 2 |
+----+------+-------+----------+
output table
+------+----------+----------+
| sent | received | order_id |
+------+----------+----------+
| 300 | 400 | 1 |
| 300 | 1100 | 2 |
+------+----------+----------+
I tried the following with no joy. This does not impose any restrictions on how I would desire to solve this problem. It is just how I attempted to do it.
SELECT *
FROM
( select SUM(value) as sent, order_id FROM sent_table WHERE name='dave' GROUP BY order_id) A
CROSS JOIN
( select SUM(value) as received, order_id FROM received_table WHERE name='dave' GROUP BY order_id) B
Any help would be greatly appreciated.

Answer 1

Do the sums on each table, grouping by order_id, then join the results. To get the rows even if one side is missing, do a FULL OUTER JOIN:
SELECT COALESCE(s.order_id, r.order_id) AS order_id, s.sent, r.received
FROM (
SELECT order_id, SUM(value) AS sent
FROM sent
GROUP BY order_id
) s
FULL OUTER JOIN (
SELECT order_id, SUM(value) AS received
FROM received
GROUP BY order_id
) r
USING (order_id)
ORDER BY 1
Result:
| order_id | sent | received |
| -------- | ---- | -------- |
| 1 | 300 | 400 |
| 2 | | 1100 |
Note the COALESCE on the order_id, so that if it's missing from sent it will be taken from recevied, so that that value will never be NULL.
If you want to have 0 in place of NULL (when e.g. there is no record for that order_id in either sent or received), you would do COALESCE(s.sent, 0) AS sent, COALESCE(r.received, 0) AS received.
https://www.db-fiddle.com/f/nq3xYrcys16eUrBRHT6xLL/2