DAY - 9
Components of E-R Diagram
The
E-R diagram has three main components.
1) Entity
An
Entity can be any object, place, person or class. In E-R Diagram, an entity
is represented using rectangles. Consider an example of an Organisation.
Employee, Manager, Department, Product and many more can be taken as entities
from an Organisation.
Weak Entity
Weak
entity is an entity that depends on another entity. Weak entity doen't have key
attribute of their own. Double rectangle represents weak entity.
2) Attribute
An
Attribute describes a property or characterstic of an entity. For
example, Name, Age, Address etc can be attributes of a Student. An attribute is
represented using eclipse.
Key Attribute
Key
attribute represents the main characterstic of an Entity. It is used to
represent Primary key. Ellipse with underlying lines represent Key Attribute.
Composite Attribute
An
attribute can also have their own attributes. These attributes are known as Composite
attribute.
3) Relationship
A
Relationship describes relations between entities. Relationship is represented
using diamonds.
There
are three types of relationship that exist between Entities.
- Binary Relationship
- Recursive Relationship
- Ternary Relationship
A.
Binary Relationship
Binary
Relationship means relation between two Entities. This is further divided into
three types.
- One to One : This type of relationship is rarely seen in real world.
The above example describes that one student
can enroll only for one course and a course will also have only one Student.
This is not what you will usually see in relationship.
- One to Many : It reflects business rule that one entity is associated with many number of same entity. The example for this relation might sound a little weird, but this menas that one student can enroll to many courses, but one course will have one Student.
The arrows in the diagram describes that one
student can enroll for only one course.
- Many to One : It reflects business rule that many entities can be associated with just one entity. For example, Student enrolls for only one Course but a Course can have many Students.
- Many to Many :
The above diagram represents that many
students can enroll for more than one courses.
B.
Recursive Relationship
When
an Entity is related with itself it is known as Recursive Relationship.
C.
Ternary Relationship
Relationship
of degree three is called Ternary relationship.
Generalization
Generalization is a bottom-up
approach in which two lower level entities combine to form a higher level
entity. In generalization, the higher level entity can also combine with other
lower level entity to make further higher level entity.
Specialization
Specialization is opposite to
Generalization. It is a top-down approach in which one higher level entity can
be broken down into two lower level entity. In specialization, some higher
level entities may not have lower-level entity sets at all.
Aggregration
Aggregration
is a process when relation between two entities is treated as a single entity.
Here the relation between Center and Course is acting as an Entity in relation
with Visitor.
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