Question

In: Computer Science

4. Consider the following two alternative ER designs: • a ternary relationship set R that relates...

4. Consider the following two alternative ER designs:

• a ternary relationship set R that relates entity sets A, B, and C.

• a design in which R is replaced by an entity set E with E related to A, B, and C by 3 binary relationship sets RA, RB and RC , respectively.

Explain why, as stated, these two may not be equivalent. Then explain what could be done to make them equivalent by fixing each of RA, RB and RC , to be 1-1. m-m, 1-m, or m-1.

Expert Solution

Solution 1:

A Ternary Relationship Set R that relates entities A,B C:

Answer: Relationship connects two objects( Entities) in a particular Context.

There are two types of Relationships:

i)Binary Relationship: Between two Entities only

ii) Ternary Relationship: Between Three Entities an Association relationship. It is a relationship derived from the basic relationship of existing two Entities A,B. That Relationship denoted as the third Entity called C.

Example: A Company(A) has Many Employees so Employee(B) is Related to his Company by Works for relation. From That relation a new entity called JOB(C) is derived and used as Job.

Solution2 :

)Design in which R is replaced by an entity set E with E related to A, B, and C by 3 binary relationship sets RA, RB and RC , respectively.

Explain why, as stated, these two may not be equivalent. Then explain what could be done to make them equivalent by fixing each of RA, RB and RC , to be 1-1. m-m, 1-m, or m-1.

Answer:

The above diagram shows the relationships between Entity A and B. B and C, A and C;

For example: An Employee( A) works for Company(B) . Multiplicity is An employee belongs to one Company( 1 – 1 from employee point of view). In special cases an employee belongs to many companies due to shifting of companies.
A company has many Employees(so Multiplicity is 1-M between Company and Employee
The relation between Entity A(Employee), Entity C(Job) is doing or Performs ( ie Employee performs job)and Multiplicity is 1-1 An employee perform only one job at a time
The relation between Entity B(Company), Entity C(Job) is offers(A Company offers many jobs) and Multiplicity is 1- M .

Both models of Problem a and b is looking varied but exhibits the same behavior.So Ternary relationship concept simplifies the data model.

venereology answered 11 hours ago

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