A relationship that is very common in a database is a many to many relationship.

In a library it is possible for

• a borrower to loan 0, 1 or MANY books (a person can be entered into the library system without actually borrowing a book) and

• a book to be loaned out 0, 1 or MANY times (a book does not need to be borrowed to be entered into the system)

It should also be noted that the same book could be borrowed out multiple times by a borrower.

So to represent this in an ERD diagram we would have

If we want to show this fully attributed as an extended ERD (E-ERD) this would look like

However in order to make this possible in a database we need to add a join table. In this case a loan table. The E-ERD diagram looks like