Once the entities are known it is important to show how they will relate to one another. There are three main relationship types

• One to one
• One to many
• Many to many

Many to many links must not exist and should have been removed during normalisation. If they had not then an extra entity must be added and two one to many links created. Also one to one links do not tend to be very useful. Although they are possible they tend to not be used much.

Once the entities are decided and all many to many links are removed then the task of drawing the diagram occurs. Consider the following tables.

Customer( CustomerID, Name)

Order( CustomerID, OrderNumber, ProductID)

Product( ProductID, Product, SupplierID)

Supplier( SupplierID, Supplier)

Customers can make more than one order. Each order can contain multiple products and finally each supplier can supply multiple products. This leads to the diagram below.