My Query Result is as below 
Query:
SELECT reservations.customerid,
(SELECT SUM(balances.balance) FROM balances
WHERE balances.customer_id = reservations.customerid)
AS Preveious_balance ,
(SELECT SUM(services.Amount_due)
FROM services
WHERE services.customer_id = reservations.customerid
AND services.status=0) AS Service_due ,
(SELECT SUM(foods.Amount_due)
FROM foods WHERE foods.customer_id = reservations.customerid
AND foods.status=0) AS Food_due,
((due_nights.Due_nights - reservations.billed_nights) * rooms.rate)
as Accomendation,
(Preveious_balance + Service_due + Food_due + Accomendation)
AS Total_due
FROM reservations, due_nights, rooms
WHERE reservations.customerid = due_nights.customerid
AND reservations.roomno = rooms.roomno
Total_due is Empty if some other columns value gets empty so how to fix this ?
Best How To :
SELECT reservations.customerid,
(SELECT SUM (balances.balance)
FROM balances WHERE balances.customer_id = reservations.customerid)
AS Preveious_balance ,
(SELECT SUM(services.Amount_due)
FROM services WHERE services.customer_id = reservations.customerid
AND services.status=0) AS Service_due ,
(SELECT SUM(foods.Amount_due)
FROM foods WHERE foods.customer_id = reservations.customerid
AND foods.status=0) AS Food_due,
((due_nights.Due_nights - reservations.billed_nights) * rooms.rate)
as Accomendation,
(NZ(Preveious_balance,0) + NZ(Service_due,0) + NZ(Food_due,0) + NZ(Accomendation,0)) AS Total_due
FROM reservations, due_nights, rooms
WHERE reservations.customerid = due_nights.customerid
AND reservations.roomno = rooms.roomno
You can you NZ function of Access
to check whether the sum is null.If so assign 0. It will solve your issue
If not access
SELECT reservations.customerid,
(SELECT IIf(Sum(balances.balance) Is Null, 0, Sum(balances.balance))
FROM balances WHERE balances.customer_id = reservations.customerid)
AS Preveious_balance ,
(SELECT IIf(Sum(services.Amount_due) Is Null, 0, Sum(services.Amount_due))
FROM services WHERE services.customer_id = reservations.customerid
AND services.status=0) AS Service_due ,
(SELECT IIf(Sum(foods.Amount_due) Is Null, 0, Sum(foods.Amount_due))
FROM foods WHERE foods.customer_id = reservations.customerid
AND foods.status=0) AS Food_due,
((due_nights.Due_nights - reservations.billed_nights) * rooms.rate)
as Accomendation,
(Preveious_balance + Service_due + Food_due + Accomendation) AS Total_due
FROM reservations, due_nights, rooms
WHERE reservations.customerid = due_nights.customerid
AND reservations.roomno = rooms.roomno