Assigning relationships in a Logical Data Model

In order to assign the relationships in a logical data model we present the following example of customer orders using the notation of Barker.

This logical model shows both how the customers place orders and how the orders are placed by customers. The verbs or sort expressions between entities (tables) identify the relationships and usually show their direction. Thus, as you can see in the figure of the logical model above customers place one or more orders and any given order is placed by one customer only. It additionally shows that a customer can live at one or more addresses and any given address can have zero or more customers who live there. In the next section we will talk about data normalization, a methodology which leads to the reduction and even to the elimination of data redundancy. This procedure is very important for database developers who must avoid storing entities or objects in a relational database which must not maintain the same information in several places.