Calculate a balance between 3 totals from different tables in FileMaker - filemaker

Question

The 2 tables in question:
INV
Expense
The 3 fields in question:
INV_total,
EXP_total,
TAX_total
I need a calculation in a report that shows the difference between the 3 fields, a bit like:
INV_total - TAX_total + EXP_total
However for some reason this relatively simple idea eludes me (certainly in terms of execution :) )
Any help would be appreciated.
I'm relatively new to FMP.
Be... Gentle...

Answer 1

There are many possible answers, depending on what exactly you need and for what for purpose. It's unlikely that you would want to total all invoices and all expenses from all times since the beginning of your solution..
Let's say you want to do this calculation for a specific year. You could have a script find the invoices of the given year, place the invoice and tax totals in variables (using summary fields to get them), move to the expenses table and do the same.
Another way would be to use a third table and get the totals through a relationship based on a date range.
Added:
Here's an expansion of the first option, using a script to get the total of expenses.
These are the fields used in the script:
Expenses:
Date (Date)
Amount (Number)
sTotalAmount (Summary, Total of Amount)
Invoices:
Date (Date)
gTotalExpenses (Number, Global)
The script itself would look something like this (here, we are producing a report for the current year):
Enter Find Mode []
Go to Layout [ Expenses ]
Set Field [ Expenses::Date ; Year ( Get (CurrentDate) ) ]
Perform Find []
Set Field [ Invoices::gTotalExpenses ; Expenses::sTotalAmount ]
Enter Find Mode []
Go to Layout [ Invoices ]
Set Field [ Expenses::Date ; Year ( Get (CurrentDate) ) ]
Perform Find []
Sort Records []
At the end of this script, you should see your report with the global gTotalExpenses field containing the total amount of expenses for the current year.

Answer 2

On a long shot I'm going to assume that the expenses relate to an invoice in this case. So to do this calculation you will be best to use some relational database skills.
Give your invoice a unique ID, the best way to do this is add a field called InvoiceID which is a unique serial number. For this example let's assume you have an invoice with an InvoiceID of 123
Add a field called InvoiceID to your Expense table. Each expense that relates to invoice with InvoiceID of 123 will need InvoiceID set to 123.
Go to the relationship graph and link InvoiceID in your invoice table to InvoiceID in your expenses table drag and drop InvoiceID in the Invoice table to InvoiceID in the expense table.
In the invoice table create a calculation field which is INVTotal - TAXTotal + sum( Expense::EXP_total )