E-R Diagrams and Types of Relationship

Basic concepts

1.         Entity sets:

An entity is a “thing” or “object” in the real world that is distinguishable from all other objects. For example, each person in an enterprise is an entity. An entity has a set of properties and the values for the some properties may uniquely identify an entity. For example, the social-security number 677-89-9011 uniquely identities one particular person in the enterprise. An entity set is a set of entities of the same type that share properties, or attributes. The set of all persons who are customers at a given bank, for example can be defined as the entity set customer.

2.         Simple and composite attribute:

The attribute that is not divided in to subparts is called Simple attribute. For example, age attributes of person entity.

The attribute divided into subparts  (i.e. other attributes for, example customer-name could be structured as a composite attributes consisting of first-name, middle-initial, and last-name.

3.         Single-valued and multi valued attributes:

An attribute having a single-value only is known as single values attribute. For example, date of birth attribute of customer entity.

An attribute having a set of values is known as multi values. For example, phone number attribute of customer entity.

4.         Derived attribute: 

The values for this type of attribute can be derived from the values of other related attributes or entities. For example, the value of age attribute of customer entity can be derived from date of birth attribute.

 Entity- Relationship Diagram:

The entity relationship (E-R) data model is based on a perception of a real world that consist of a collection of basic objects, called entities, and of relationships among these objects. Entities are described in a database by a set of attributes. A relationship is an association among several entities. The relative simplicity and pictorial clarity of this diagramming technique may well account in large part for the widespread use of the E-R model. Such a diagram consists of the followings major components:

Rectangles:              which represent entity sets     Ellipses:                    which represent attributes Diamonds:                which represent relationship sets Lines:                         which link attributes to entity sets and entity to relationship sets Double ellipses:       which represent multi-valued attributes Dashed ellipse:        which denote derived attributes Double lines:            which indicated total participation of an entity in a relationship sets

There Are Four Types Of Relationship:

            - One to One

            - One to Many

            - Many to One

            - Many to Many

As depicted in Figure , attributes of an entity that are members of primary key are underlined.

Consider the entity-relationship diagram in Figure , which consists of two entity sets of customers and loan, related thought a binary relationship sets borrower. The attributes associated with customer are customer-name, social-security, customer-street and customer-city. The attributes associated with loan are loan-number and amount.     

The relationship set borrower may be many-to-many, one-to-many, many-to-one, or one-to-one. To distinguish among these types, we draw either a directed line (-->) or an undirected line (--) between the relationship set and the entity set in question.

A directed line from the relationship set borrower to the entity set loan specifies that either borrower is a one-to-one, or many-to-one relationship set, from customer to loan; borrower cannot be a many-to-many or a one-to-many relationship set, from customer to loan.

An undirected line from the relationship set borrower to the entity set loan specifies that either borrower is a many-to-many, or one-to-many relationship set, from customer to loan.

Returning to the E-R diagram of figure, we see that the relationship set borrower is many to many. If the relationship set borrower were one to many, from customer to loan, then the line from borrower to customer would be directed, with an arrow pointing to the customer entity set (figure). Similarly, if the relationship set borrower were many to one from customer to loan, then the line from borrower to loan would have an arrow pointing to the loan entity set (Figure ). Finally, if the relationship set borrower were one to one, then both lines form borrower would have arrows: one pointing to the loan entity set, and one pointing to the customer entity set (Figure ).