The E-R Model

1. The entity-relationship model is based on a perception of the world as consisting of a collection of basic objects (entities) and relationships among these objects.
   • An entity is a distinguishable object that exists.
   • Each entity has associated with it a set of attributes describing it.
   • E.g. number and balance for an account entity.
   • A relationship is an association among several entities.
   • e.g. A cust_acct relationship associates a customer with each account he or she has.
   • The set of all entities or relationships of the same type is called the entity set or relationship set.
   • Another essential element of the E-R diagram is the mapping cardinalities, which express the number of entities to which another entity can be associated via a relationship set.
   We'll see later how well this model works to describe real world situations.

2. The overall logical structure of a database can be expressed graphically by an E-R diagram:
   • rectangles: represent entity sets.
   • ellipses: represent attributes.
   • diamonds: represent relationships among entity sets.
   • lines: link attributes to entity sets and entity sets to relationships.
   See figure 1.2 for an example.

     
   Figure 1.2: A sample E-R diagram.