slide 6 er strong & weak entity
DESCRIPTION
strong & weak entityTRANSCRIPT
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
ER MODELING
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Strong & Weak
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Notations:
Identifying Relationship
Total Participation
Weak Entity Set
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Weak Entity :
Entity that cannot be uniquely identified by its attributes alone. Therefore it must use a foreign key in conjunction with its attribute to create a primary key.
Definition : An entity set may not have sufficient attributes to form a primary key.
Strong Entity :Definition : An entity set that has a primary key is termed as a strong entity set.
Weak Entity :
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Loan Payment
Loan#
Amount Payment_amount
Payment_no
Payment_date
LoanPayment
1 N
Discriminator
Owner Entity Set Identifying Relationship
Weak Entity Set
Total Participation
LoanPayment
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Identifying or Owner Entity Set : For a weak entity Set to be meaningful, it must be associated with another entity set called identifying or owner entity set.
Identifying Relationship : The relationship associating the weak entity set with the identifying relationship.
: It is many to one from the weak entity set to owner entity set.
: The participation of the weak entity set in the relationship is total.
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Discriminator : It is a set of attributes that allows distinguishing among all those entities in the weak entity set that depend on one particular strong entity.
: It is also called partial key of the weak entity set.
Primary key of the weak entity set
Primary key of the Owner entity set
Discriminator of the weak entity set
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Note : The identifying relationship set should have no descriptive attributes.
Professor DepartmentLoanPayment
Since
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Properties :
A Weak Entity set can participate in relationship other than the identifying relationship.Ex : payment & account
A Weak entity set may participate as owner in an identifying relationship with another weak entity set.
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
It is also possible to have a weak entity set with more than one identifying entity set.
Then
Primary key of the weak entity set
Union of Primary keys of the Owner entity set
Discriminator of the weak entity set
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Total Participation: Every entity in the set is involved in some association (or tuple) of the relationship
Partial Participation: Not all entities in the set are involved in association (or tuples) of the relationship
Notation :
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Recursive Relationships and Role Names
•Recursive relationship: An entity set relating to itself gives rise to a recursive relationship
•Role Names –used to specify the exact role in which the entity participates in the relationships
•Essential in case of recursive relationships•Can be optionally specified in non-recursive cases
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW
Prepared by Visakh V, Assistant Professor,Dept. of CSE, LBSITW