An INVOICE is written by a SALESREP. Each sales representative can write many invoices, but
each invoice is written by a single sales representative.
The INVOICE is written for a single CUSTOMER. However, each customer can have many
invoices.
An INVOICE can include many detail lines (LINE), each of which describes one product
bought by the customer.
The product information is stored in a PRODUCT entity.
The product’s vendor information is found in a VENDOR entity.