SQL return row when sum value is null - sql

Question

I got two tables. One with a bill of material and one with purchasing orders. Now I want to display the full bill of material of a product with the total on order amounts from the table purchasing.
**Billofmaterials**
BOM_ID BOM_Prod_id BOM_item_id
1 5 11
2 5 13
3 6 11
4 6 15
5 6 20
Example prod_id (product id) 6 has 3 items (id 11, 15 and 20).
**Purchasing**
PU_ID PU_item_id PU_amount PU_status
1 11 100 On order
2 11 650 On order
3 11 40 Received
4 20 600 On order
5 8 10 On order
6 15 150 Received
Now i got the following SQL
SELECT
BOM_item_id,
SUM(DISTINCT purchasing.PU_amount) as total_on_order
FROM Billofmaterials
LEFT JOIN purchasing
ON Billofmaterials.BOM_item_id= purchasing.PU_item_id
AND purchasing.PU_status != 'Received'
AND BOM_prod_id = 6
GROUP BY BOM_item_id
This query returns the following:
**Query result**
BOM_item_id total_on_order
11 750
20 600
Because there is only one received purchase order for BOM_item_id 15 it doesn't return a value. Now i want to retun BOM_item_id 15 also but with a total_on_order as 0 like:
**Expected result**
BOM_item_id total_on_order
11 750
15 0
20 600
What SQL feature/function do I need to use to get the expected result?

Answer 1

You can try the below -
SELECT BOM_item_id,coalesce(total_on_order,0) as total_on_order
FROM Billofmaterials left join
(
select PU_item_id,SUM(purchasing.PU_amount) as total_on_order
from purchasing
where purchasing.PU_status != 'Received'
group by PU_item_id
) purchasing
ON Billofmaterials.BOM_item_id= purchasing.PU_item_id
where BOM_prod_id = 6