Customer would be one table (Clients, People, Contacts) Customer has many Invoices
Invoice would be one table - Invoices contain many Products
Invoice LineItem would be one table - LineItem belongs to one invoice and one product
Products would be one table (Products, Rentals, Services) Products will be on many Invoices
You are renting products with prices - hammeroneday, hammertwoday, hammersevenday.
The simpler you make the database (called normalization - normalize) the simpler to create more complexity as you become used to the database.
The first five links in the list here would be useful reads.
Thank you for your response. Please let me explain this in a little more detail. I am using a table for Customers, which has many invoices, which in turn has many products. For this particular purpose - renting 3 items - we have chosen not to use the extra table for the many products on a invoice, but to use 3 fields on the invoice itself instead.
My challenge is, how to select a value in the right row of a 3-row table, with the right column as a function of the number of days which is selected to rent an item. Hope that this makes sense?