I decided to scrap my customer accounts database and go back to the beginning. So now I have a new database which consists of three main tables: Customers, Payments, Bookings. A one-to-many relationship exists from Customers into both Payments and Bookings, with Customers on the one end.
The data is in one table, Customer Data. I have been trying to append certain rows from Customer Data to each of the three new tables, with no success - every record gets a key violation.