# Database Systems

Topics: Data modeling / Pages: 4 (817 words) / Published: Aug 3rd, 2013
The entity-relationship model (or ER model) is a way of graphically representing the logical relationships of entities (or objects) in order to create a database. entity type is a collection of entity instances sharing similar properties
Strong Entity Vs Weak Entity
An entity set that does not have sufficient attributes to form a primary key is termed as a weak entity set. An entity set that has a primary key is termed as strong entity set.

A weak entity is existence dependent. That is the existence of a weak entity depends on the existence of a identifying entity set. The discriminator (or partial key) is used to identify other attributes of a weak entity set.The primary key of a weak entity set is formed by primary key of identifying entity set and the discriminator of weak entity set. The existence of a weak entity is indicated by a double rectangle in the ER diagram. We underline the discriminator of a weak entity set with a dashed line in the ER diagram.

Recursive Entity
A recursive entity is one in which a relation can exist between occurrences of the same entity set. This occurs in a unary relationship.

Composite Entities

If a Many to Many relationship exist we must create a bridge entity to convert it into 1 to Many. Bridge entity composed of the primary keys of each of the entities to be connected. The bridge entity is known as a composite entity. A composite entity is represented by a diamond shape with in a rectangle in an ER Diagram.
Types of Relationshipso Unary RelationshipAn ENTITY TYPE linked with itself, also called recursive relationship. ExampleRoommate, where STUDENT is linked with STUDENT
Binary relationshipA Binary relationship is the one that links two entities sets e.g. STUDENT-CLASS.Relationships can be formally described in an ordered pair form
Ternary RelationshipA Ternary relationship is the one that involves three entities e.g.STUDENT-CLASS-FACULTY N-ary RelationshipMost relationships in data model are binary or