subquery the same table in select statement - sql

Question

I have a resturant db and I need to total up the total value of all the items sold individually. So if I sold a hamburger that has a base price of $10.00 with bacon which costs $1.00 and a hambuger(again $10.00) with avacado that costs $0.50 I need to get $21.50 returned. My invoice table looks like this:
invoice_num item_num price item_id parent_item_id
111 hmbg 10.00 guid_1 ''
111 bacn 1.00 guid_2 guid_2
112 hmbg 10.00 guid_3 ''
112 avcd 0.50 guid_4 guid_3
I can get the sum of all the parent items like this:
SELECT item_num, SUM(price) FROM invoices WHERE parent_item_id = ''
it is the adding of the toppings that is confusing me. I feel like I need to add a subquery in the SUM but I'm not sure how to go about doing it and referencing the original query to use the item_id.

Answer 1

SELECT item_num, sum(i.price) + sum(nvl(x.ingred_price,0))
FROM invoices i
LEFT OUTER JOIN
(SELECT parent_item_id
, sum(price) ingred_price
FROM invoices
WHERE parent_item_id IS NOT NULL
GROUP BY parent_item_id) x
ON x.parent_item_id = i.item_id
WHERE i.parent_item_id IS NULL
GROUP BY item_num
Here's a SQL Fiddle that proves the above code works. I used Oracle, but you should be able to adapt it to whatever DB you are using.
Assumption: You don't have more than one level in a parent child relationship. E.g. A can have a child B, but B won't have any other children.

Answer 2

Not clear based on your question (see my comment) but as I understand it a simple group by will give you what you want. If not please explain (in the original question) why does this query does not work --- what is it missing from your requirements?
SELECT item_num, SUM(price)
FROM invoices
GROUP BY item_num

Answer 3

Hard to say, but looks like you need recursive cte.
Here's example for PostgreSQL:
with recursive cte as (
select
t.invoice_num, t.price, t.item_id, t.item_num
from Table1 as t
where t.parent_item_id is null
union all
select
t.invoice_num, t.price, t.item_id, c.item_num
from Table1 as t
inner join cte as c on c.item_id = t.parent_item_id
)
select invoice_num, item_num, sum(price)
from cte
group by invoice_num, item_num
sql fiddle demo
I've used null for empty parent_item_id (it's better solution than using empty strings), but you can change this to ''.