Chen ERD: What improvement could I make? Can relationship `RECEIVES` be omitted since entity `INVOICE` is a weak entity of `CAR`?

Question

Assumptions:

• A salesperson may sell many cars, but each car is sold by only one salesperson.

• A customer may buy many cars, but each car is bought by only one customer.

• A salesperson writes a single invoice for each car he or she sells.

• A customer gets an invoice for each car he or she buys.

• A customer may come in just to have his or her car serviced; that is, a customer need not buy a car to be classified as a customer.

• When a customer takes one or more cars in for repair or service, one service ticket is written for each car.

• The car dealership maintains a service history for each of the cars serviced. The service records are referenced by the car’s serial number.

• A car brought in for service can be worked on by many mechanics, and each mechanic may work on many cars.

• A car that is serviced may or may not need parts (e.g., adjusting a carburetor or cleaning a fuel injector nozzle does not require providing new parts).

Can relationship RECEIVES be omitted since entity INVOICE is a weak entity of CAR?

Answer 1

It's amazing how many people engage in pure imagination - complete and utter fantasy - about how various kinds of business actually do business.

In any business I've worked in, a sale of goods results first in a sales order (which records that the business has entered into legal relations for the delivery of goods for a price, and has accrued both rights and obligations).

A sales invoice is not something related to the car at all, but to the account between the business and the customer. A sales invoice calls for the customer to make a payment on the account.

There is certainly no strict 1-to-1 relationship between cars and invoices.

Also, invoices are not "weak entities" of anything else, because they are uniquely identifiable by an invoice number. The law typically dictates the form that invoices must take, and one of the criteria is that invoices must be generated with a unique and sequential number.

As a final point, you are making a weak and unclear distinction in your thinking between business records (like invoices and service tickets), and real/physical things like cars and people, and conflating many roles.

In practicable systems, the invoice is often not written by a salesperson at all. It is either written by an accounts clerk, or by the computer under the direction of a software developer.

And the salesperson does not sell the car. The car dealer business sells the car, and the salesperson acts as an agent of the business.

The actual relationships are more like a clerk drafts a sales order (a kind of business record), and a sales order has identifiers for the car, the customer, and the sales rep.

But the car never "has an order" - it's the order that has a car identifier. It's the business record that has information about real things or data which associates it with real things, not the other way around.