Total amount of financing already provided to each customer - sql

Question

I have three separate tables:
SystemA_Cash
SystemA_Loans
SystemB_CarLoans
Each table has Cash_AdvanceID field, Customer, & Funded Amount.
I need to total the amount funded for each customer.
Each customer can be present in any of the tables too.
So far I did
SELECT SystemA_CashAdvances.CashAdvanceID, SystemA_CashAdvances.Customer,
SystemA_CashAdvances.Funded_Amount
FROM
SystemA_CashAdvances
INNER JOIN
SystemA_Loans ON SystemA_Loans.CashAdvanceID = SystemA_CashAdvances.Cash_AdvanceID
INNER JOIN
SytemB_CarLoans ON SystemA_CashAdvances.Cash_AdvanceID = SystemB_CarLoans.CashAdvanceID;
It doesn't seem to return one table with the customers and the total amount each was funded.

Answer 1

You can use UNION and GROUP BY for this, like:
SELECT Cash_AdvanceID,
Customer,
SUM(Funded_Amount) AS "Total_Funded_Amount"
FROM (
SELECT Cash_AdvanceID,
Customer,
Funded_Amount
FROM SystemA_CashAdvances
UNION
SELECT Cash_AdvanceID,
Customer,
Funded_Amount
FROM SystemA_Loans
UNION
SELECT Cash_AdvanceID,
Customer,
Funded_Amount
FROM SytemB_CarLoans
)
GROUP BY Cash_AdvanceID,
Customer;
To do this with JOIN will be hard, as MS Access does not support a full outer join, as other database engines do. The closest is a one-sided outer join, as follows:
SELECT a.Cash_AdvanceID,
a.Customer,
a.Funded_Amount + Nz(l.Funded_Amount)
+ Nz(c.Funded_Amount) AS "Total_Funded_Amount"
FROM (SystemA_CashAdvances AS a
LEFT JOIN SystemA_Loans AS l
ON a.Cash_AdvanceID = l.Cash_AdvanceID)
LEFT JOIN SytemB_CarLoans AS c
ON a.Cash_AdvanceID = c.Cash_AdvanceID;
The above will work for customers that appear at least in the table SystemA_CashAdvances. If they only appear in another table, they will be excluded from the result. To get those in as well, you would need to use UNION again, which brings us back to the first solution above.