Nested self join and creating multiple sums - sql

Question

I have a basic parent / child scheme for expenditures:
The underling data is the same so I just added a category column and parent_id. These have child records:
I am trying to aggregate the totals form the orders, related orders and difference between the two like this:
Which is grouped by the orders overall then I am also looking for something like this:
I can get the order_amount no problem either way. That's a simple JOIN and SUM.
I am stuck on the secondary JOINS given that I have to JOIN the invoices expenditures to the orders then JOIN the invoice expenditure items and SUM that up.
I am looking for direction on the correct JOIN or if there is a better way to approach this with some sort of subquery etc.

Answer 1

To sump up by order, one solution would be to use a conditional aggregate query. A trick is to check the category to decide whether to use the value from column expenditures.id or from column expenditures.parent_id as grouping criteria:
SELECT
CASE WHEN e.category = 'order' THEN e.id ELSE e.parent_id END expenditure_id,
SUM(CASE WHEN e.category = 'order' THEN i.amount ELSE 0 END) order_amount,
SUM(CASE WHEN e.category = 'invoice' THEN i.amount ELSE 0 END) order_amount,
SUM(CASE WHEN e.category = 'order' THEN i.amount ELSE 0 END)
- SUM(CASE WHEN e.category = 'invoice' THEN i.amount ELSE 0 END) balance
FROM expenditures e
LEFT JOIN expenditure_items i ON e.id = i.expenditure_id
GROUP BY CASE WHEN e.category = 'order' THEN e.id ELSE e.parent_id END
ORDER BY expenditure_id
Demo on DB Fiddle:
| expenditure_id | order_amount | order_amount | balance |
| -------------- | ------------ | ------------ | ------- |
| 1 | 3740 | 0 | 3740 |
| 2 | 11000 | 9350 | 1650 |
The second query, that sums up by item code, basically follows the same logic, but groups by idem code instead:
SELECT
i.code,
SUM(CASE WHEN e.category = 'order' THEN i.amount ELSE 0 END) order_amount,
SUM(CASE WHEN e.category = 'invoice' THEN i.amount ELSE 0 END) order_amount,
SUM(CASE WHEN e.category = 'order' THEN i.amount ELSE 0 END)
- SUM(CASE WHEN e.category = 'invoice' THEN i.amount ELSE 0 END) balance
FROM expenditures e
LEFT JOIN expenditure_items i ON e.id = i.expenditure_id
GROUP BY i.code
ORDER BY i.code;
Demo:
| code | order_amount | order_amount | balance |
| ---- | ------------ | ------------ | ------- |
| a | 13400 | 8500 | 4900 |
| b | 1340 | 850 | 490 |