SHORT SUMMARY OF THE PROJECT
INTRODUCTION:ER model refers to the 'Entity-Relationship' model.It is actually a design(or blueprint)of the database which can later be implemented as a database.It describes the inter-related things in a specific domain of knowledge.
COMPONENTS OF ER MODEL:It consists of the following components:
- Entity and entity sets
- Relationship set
ENTITY AND ENTITY SETS:An object in the physical world is referred to as an 'Entity'.
An entity can also have a conceptual existence
E.g.Employee's company,student's university course,etc.
A set of all entities is referred to as an 'Entity set'.
Consider an example of Employee entity set,in this,E1,E2,E3 are the entities of type 'Employee', and the set of all employees together forms the 'Entity set'.
The pictorial representation (ER diagram)is given below:
Each entity is described by many other properties called ‘attributes'.
For an entity type ‘Employee', the attributes are E Id,E name,E dept,E phno,E doj,etc.
In an ER diagram,the attributes are represented by an ‘oval' symbol as follows.
TYPES OF ATTRIBUTES: An attribute can be of the following types:
KEY ATTRIBUTE:An attribute among the set of attributes which uniquely identifies each entity in the entity set is called the 'key attribute'.
For example, E id will be unique for each employee.
In the ER diagram,the key attribute is represented by an oval symbol with an underline as shown below:
COMPOSITE ATTRIBUTE: An attribute consisting of many other attributes is called a 'composite attribute'.
For example, 'Address' attribute of Employee entity type consists of the Hno,street,city,state,etc.
In the ER diagram,a composite attribute is represented by an oval comprising of other symbols.
MULTI-VALUED ATTRIBUTES: A multi-valued attribute is an attribute comprising of more than one value for a given entity.
For example, E phno, i.e.; an employee can have morethan one phone number.
In the ER diagram,a multi-valued attribute is designated by double oval symbol.
DERIVED ATTRIBUTE: An attribute that can be taken from other attributes of the same entity type is referred to as a 'derived attribute'.
For example, Employees age which can be derived from the employee's date of birth.
In the ER diagram,a derived attribute is designated by a dashed line.
Hence,the ER diagram of the entity type 'Employee'is represented as follows:
RELATIONSHIP SET: The association among the entity types is represented by relationship type. For example" employed in" is a relationship type that exists among entity type 'Employee' and 'Dept.'
In the ER diagram, a relationship type is designated by a diamond symbol and the entities are connected using lines.
A Relationship set is termed as a set of relationships of the same type. The following relationship set shows.
Employee1 is employed in dept3, Emp2 is employed in dept2 and employee3 is employed in dept 1.
DEGREE OF A RELATIONSHIP SET: The total number of distinct entity sets which participates in a relationship set is called the degree of the relationship set.
TYPES OF RELATIONSHIPS:
UNARY RELATIONSHIP: When only one entity set is participating in a relation, the relationship is known as 'Unary relationship.'
For example, an employee can work only in one department
BINARY RELATIONSHIP: When two entities are participating in a relation, the relationship is known as 'binary relationship.'
For example, an employee works in only one department of a company;
n-ARY RELATIONSHIP:When n entities are participating in a relation, the relation is known as an 'n-ary relationship.'
CARDINALITY: It refers to the number of times an entity of an entity set is participating in a relationship set.It can be of the following types:
One-to-one: When each entity is participating only one time in the relationship, the cardinality is said to be one-to-one.
For example, only one employee can be awarded as the 'best employee' of the Department i.e.; each department has one employee awarded as the best employee.
MANY -TO -ONE: When entities of one entity set participate only once in the relationship set and the entities of another entity set are participating more than once in the relationship set, the cardinality is said to be many to one.For example, an employee can work in only one dept. but, a dept. can have many employees.
MANY TO MANY: When entities of all the entity sets are participating more than once in the relationship, the cardinality is said to be many to many.
For example, an employee can work on two or more projects, and many employees can accomplish each project.
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