How to query a custom record field

Hi! I was trying to use a query to obtain a value from a database. Concretely, I want to return the value of a custom record field, with some conditions/filters (the Purchase Order ID should be equal to the one used in the process, and the Status of the Invoice should be accepted). All of this are conditions based in the same record type.

I had to use the custom record field (total invoice price, Record 1) to obtain the sum of the price of each invoice line (Record 2). 

As I said,  I have not only source fields in my records, but also custom record fields.

The problem here is that I can´t refer to the custom record field (pointed in red in the last picture) via query, because that fields are not shown in the database (cloud console). And the querys directly point to the database. 

So, what would you recommend me in order to get the value of the custom record field? Is it possible to add the custom record field to the database, in order to make it possible to query it?

Thanks a lot