Database Management System

1. Strong and Weak Entity
The strong entity is the one whose existence does not depend on the existence of any other entity in a
schema. It is denoted by a single rectangle. Two strong entity’s relationship is represented by single
diamond. A strong entity always has the primary key in the set of attributes that describes the strong entity.
It indicates that each entity in a strong entity set can be uniquely identified.
The entity which does not have sufficient attributes to form a primary key is called as Weak entity. Weak
entity is depend on strong entity to ensure the existance of weak entity. Weak entity is represented by
double rectangle. The relation between one strong and one weak entity is represented by double diamond.
Fig 1: The relationship between weak entity and strong entity
Difference between Strong and Weak Entity:
S.NO STRONG ENTITY WEAK ENTITY
1.
Strong entity always has primary
key.
While weak entity has partial discriminator
key.
2.
Strong entity is not dependent of
any other entity.
Weak entity is depend on strong entity.
3.
Strong entity is represented by
single rectangle.
Weak entity is represented by double
rectangle.
4.
Two strong entity’s relationship is
represented by single diamond.
While the relation between one strong and
one weak entity is represented by double
diamond.
5.
Strong entity have either total
participation or not.
While weak entity always has total
participation.

2. Degree of relationship
Degree of relationship is the number of entity sets that are participated (associated) in that relationship.
That is, the number of entity sets that are connected through the relationship in question is called the degree
of relationship.
Based on the degree, the relationships may be identified as unary (degree 1), binary (degree 2), ternary
(degree 3) and so on.
A unary relationship is when both participants in the relationship are the same entity.
For Example: Subjects may be prerequisites for other subjects.
A binary relationship is when two entities participate, and is the most common relationship degree.
For Example:
A ternary relationship is when three entities participate in the relationship.
For Example: The University might need to record which teachers taught which subjects in which courses.
Extended E-R Features
Generalization
Generalization is the process of extracting common properties from a set of entities and create a
generalized entity from it. It is a bottom-up approach in which two or more entities can be generalized to a
higher level entity if they have some attributes in common. In generalization, the higher level entity can
also combine with other lower level entities to make further higher level entity.

3. 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. The idea behind Specialization is to find the subsets of entities
that have few distinguish attributes.
For example – Consider an entity employee which can be further classified as sub-entities Technician,
Engineer & Accountant because these sub entities have some distinguish attributes.

4. Aggregration
Aggregation is a process in which a single entity alone is not able to make sense in a relationship so the
relationship of two entities acts as one entity.
In the diagram above, the relationship between Center and Course together, is acting as an Entity, which
is in relationship with another entity Visitor. Now in real world, if a Visitor or a Student visits a Coaching
Center, he/she will never enquire about the center only or just about the course, rather he/she will ask
enquire about both
A. S. M. Shafi
Lecturer
Department of Computer Science and Engineering
Khwaja Yunus Ali University
Enaytpur, Sirajgonj-6751, Bangladesh