Thanks for your comment. I have not posted all code because the other checks that are performed are similar to the two types that I mentioned above.

Some more information:
The form consists of
- editable QLineEdits (easy to verify)
- a QTableView containing a QStandardItemModel called "tempinvoiceentries" (see code)
- some non-editable QLineEdits that are filled with customer data after a customer is selected from a popup with a QTableView containing a QSqlTableModel with the customer data (after the call the variable CustomerKey is set, see code)

I would change your return value too, returning 0 (FALSE) for OK seems a little strange.
I am confused...I thought all applications return 0 on success, why should this be different while using functions?

Thanks for the hint to use

Qt Code: 
  1. CREATE PROCEDURE isInvoiceEntriesPresent
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This is indeed a good possibility to improve the code and will make things easier, I was not aware of this. Also thanks for the hint to create an enum, so much to learn

What exactly is the difference between COUNT(*) and COUNT(1) ?

I read the sqlite docs on this
count(X)
The count(X) function returns a count of the number of times that X is not NULL in a group.
but I don't unterstand what this exactly means.