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How to subtract two records in a portal

Question asked by tangey on May 25, 2018
Latest reply on May 28, 2018 by onefish

I am brand new to database and filemaker. I have a project which comprises a customer table, a premises table, and a "readings"  table which has 30 fields, each field is a numeric value that represents a non-resettable meter. At set intervals, (say one per week), a new reading of the meters is taken. Each meter will either be the same or greater than the last reading.


I have gotten my project down to a portal that will only contain two row from the readings table, each row being a reading from a different date. I now need someway of doing a subtraction across all the fields, so as I can see how each field has changed between the two dates. Can someone pease advise the best way of proceeding. I have identified the unique primary key for each of the two table entries.