E-R Diagram in DBMS

 

E-R Diagram in DBMS

An Entity-Relationship model (ER model) describes the structure of a database with the help of a diagram, which is known as Entity Relationship Diagram. An ER is a design or blueprint of a database that can later be implemented as a database. An ER diagram shows the relationship among different entity sets.

It consists of three main components:

i)  Entity

ii) Attributes

iii) Relationship

 

It is a GUI representation of the logical structure widely used in database designing. It helps to identifying the relationships between/among entities in a database.

 

Components of ER Diagram

ER Diagram has three main components: -

1. Entity: It is an object or component of a data represented by a rectangle.

For example, in the above ER Diagram there are two entities one is student and other is course.

There are two type of entities i.e., weak entity and strong entity

Weak Entity: An entity which cannot be uniquely identifies or doesn't have it own key. It is dependent on relationship with some other entity. It is represented by double rectangle. For example, an apartment cannot exist without the building i.e., entity apartment is dependent on the entity building.

                                          Strong Entity and Weak Entity Example

 Strong Entity:- A strong entity can be uniquely identified. It has its own primary key and represented by rectangle and primary key is represented by a underline symbol. In the above given diagram building is a strong entity which can be uniquely identified by a building number.

 2. Attributes :An attribute defines the property of an entity. It is represented by a oval shape. for example in the picture (i) student roll no., name, date of birth are attributes of a student.

There are basically four types of attributes:

i)  Key attribute:   a key attribute can uniquely identify a entity. text of key attribute is underlined. for example, roll no. of a student can uniquely identifies a student from a class of students.

ii)  Composite attribute: A composite attribute is a combination of different attributes. for example, in entity student address is composed of state, city, pin code, flat no.

iii) Multivalued attribute: An attribute that can have multiple values is known as a multivalued attribute. It is represented by a double oval. For example, a student can have multiple addresses and multiple phone numbers.

iv) Derived attribute: an attribute whose value can be derived from another attribute is known as a derived attribute and represented by a dashed oval. for example, age of a student can be derived from his date of birth.

 

3. Relationship :- It represents relation between/among entities and represented by a diamond.

Cardinality : It defines the numerical attributes of the relationship between two entities and entity sets. There are four types of cardinalities:

i)  One to One Relationship: when a single instance of an entity is associated with the single instance of another entity then it is known as one-to-one relationship. For example, a person can hold only a single PAN card.

                                      

ii) One-to-many Relationship: when a single instance of an entity is associated with the multiple instances of another entity then it is known as one-to-many relationship. For example, A publisher can supply many books.

                   

iii) Many to one Relationship: when more than one instance of an entity is associated with the single instance of another instance then it is known as Many to one Relationship. For example, many students can enroll in a single course.

iv) Many to Many Relationships: when more than one instances of an entity is associated with more than one instances of other entity then it is known as many to many relationships. for example, a course can be assigned to many students and many students can take a single course(see the above example).