Subtracting the total quantity of table 1 to table 2

Question

Guys i have this problem on subtracting the the two tables (Deliveries and Returns) by its TOTAL_QUANTITY grouped by their CONTAINER_TYPE and CUSTOMER_NAME, this sample query returns the column Customer_Balance of 0 if I subtracted for example 5 - 0, but if i subtracted 5 -1 it returns 4.

select TOTAL_DELIVERY.CUSTOMER_NAME,
    TOTAL_DELIVERY.CONTAINER_TYPE,
    coalesce(TOTAL_DELIVERY.TOTAL_QUANTITY, 0) as TOTAL_DELIVERY,
    coalesce(TOTAL_PULLOUT.TOTAL_QUANTITY, 0) as TOTAL_PULLOUT,
    coalesce((TOTAL_DELIVERY.TOTAL_QUANTITY - TOTAL_PULLOUT.TOTAL_QUANTITY), 0) as CUSTOMER_BALANCE
from TOTAL_DELIVERY
left join TOTAL_PULLOUT
on TOTAL_DELIVERY.CUSTOMER_NAME = TOTAL_PULLOUT.CUSTOMER_NAME
and TOTAL_DELIVERY.CONTAINER_TYPE = TOTAL_PULLOUT.CONTAINER_TYPE


Deliveries Table (View)
================
----------------------------------------------
CUSTOMER_NAME| CONTAINER_TYPE | TOTAL_QUANTITY
-------------+----------------+---------------
Bryan        | Slim           | 5
-------------+----------------+---------------
Bryan        | Jug            | 5
-------------+----------------+---------------


Returns Table (View)
=============

CUSTOMER_NAME| CONTAINER_TYPE| TOTAL_QUANTITY
-------------+---------------+---------------
Bryan        | Slim          | 5
-------------+---------------+---------------

Expected output
===============

Customer | Container | Total_Delivery | Total_Return | Customer_Balance |
---------+-----------+----------------+--------------+------------------ 
Bryan    | Slim      | 5              | 5            | 0                |
---------+-----------+----------------+--------------+------------------
Bryan    | Jug       | 5              | 0            | 5                |
---------+-----------+----------------+---------------------------------

My INCORRECT Result output (incorrect result at the bottom right cell)
=========

Customer | Container | Total_Delivery | Total_Return | Customer_Balance |
---------+-----------+----------------+--------------+------------------ 
Bryan    | Slim      | 5              | 5            | 0                |
---------+-----------+----------------+--------------+------------------
Bryan    | Jug       | 5              | 0            | 0                |
---------+-----------+----------------+---------------------------------

can you please tell me what am i doing wrong? thank you!

Answer 1

(TOTAL_DELIVERY.TOTAL_QUANTITY - TOTAL_PULLOUT.TOTAL_QUANTITY)

must be

(coalesce(TOTAL_DELIVERY.TOTAL_QUANTITY,0) - coalesce(TOTAL_PULLOUT.TOTAL_QUANTITY, 0))