The entity-relationship (E-R) data model is based on a perception of a real world that consists of a collection of basic objects, called （71），and of relationships among these objects. An entity is a “thing” or “object” in the real world that is distinguishable from other objects. Entities are described in a database by a set of （72） . A relationship is an association among
several entities. The set of all entities of the same type and the set of all relationships of the same type are termed an entity set and relationship set, respectively. The overall logical structure (schema) of a database can be expressed graphically by an E-R diagram, which is built up from the following components: （73） represent entity set, （74） represent attributes, etc. In addition to entities and relations, the E-R model represents certain（75） to which the contents of a database must conform. The entity-relationship model is widely used in database design.
