Mirza Zeeshan Baig
Roll No:
Class:
Semester:
Subject:
20
BSCS
4th
DataBase System
Recursive Relationship Type
When there is a relationship
between two entities of the
same type, it is known as a
recursive relationship. This
means that the relationship is
between different instances of
the same entity type.
Some examples of recursive
relationship can be shown as
follows .
Some Examples of
Recursive Relationship
A student can be a class monitor and
handle other students. Hence, this is a
recursive relationship of entity student
with itself. This is a 1 to many recursive
relationship as one student is monitor
of many students.
A person can have many children
who are also persons. Hence, this is
a recursive relationship of entity
person with itself. This is a 1 to
many recursive relationship as one
person can be parent of many
persons.
Weak Entity
Definition:
An entity set may not have sufficient attributes to form a primary
key.
•
Entity cannot be uniquely identified by its attributes alone.
• Therefore, it must use a foreign key in conjunction with its attributes to
create a primary key.
• It is denoted with a rectangle shape.
• Weak entity cannot exist without the identifying relationship.
Identifying Relationship
Definition:
The Relationship associating the weak entity set
with the identifying relationship.it is denoted with a diamond
shape.
It is many to one from the weak entity set to the owner
entity set.
Owner Entity Type:
It is type of entity type that related to
weak entity type through conjunction of some their
attributes value.
For a weak entity set to be meaningful, it must be
associated with another entity set called owner entity set.
Partial Key
Partial Key:
Partial is a set of attributes that uniquely
identify weak entities that are related to same owner entity.
Weak entity do not have primary key attributes of their
own.They may have partial key.
Diagram identifying
Relationship
The End