Database -Entity-Relationship Model 2012/03/05

Entity-Relationship Model The ER model describes data as entities, relationships, and attributes. o Entity ： a thing in the real world with an independent existence o Attribute ： a particular property of an entitiy o Entity type: a collection of entities that have the same attributes

Entity-Relationship Diagram o Entity type: an entity type is represented as a rectangular box enclosing the entity type name. o Weak entity type: An entity type has an existence dependence with respect to its identifying relationship. A weak entity type is distinguished by surrounding its box with double lines.

Entity-Relationship Diagram o Attribute: an attribute is represented as an oval enclosing the attribute name and attached to its entity type by straight line. o Multivalued attribute: an multivalued attribute is distinguished by surrounding its oval with double lines. o Composite attribute: an composite attribute can be divided into smaller subparts and attached to the original attribute. o Key attribute: An entity type usually has an attribute whose values are distinct for each individual entity in the entity set. Such an attribute is called a key attribute and has its name underlined inside the oval.

Example