1. Database Management System
Lecture - 9

2. Relationships

3. Relationships
Relationships are the connections and interactions between the entities instances, e.g., Program and Student ETs are linked
How to identify relationships

4. Naming Relationships Up to you
If there is no proper name of the association in the system then participants’ names of abbreviations are used

5. Naming STUDENT and CLASS have ENROLL relationship
However, it can also be named as STD_CLS

6. Symbol for Relationships
DEPT
EMP
BOOK
STD

7. Relationships Relationship type can be identified like an entity type
A relationship type is an abstraction of a relationship

8. Relationships
Entities involved in a relationship are called its participants
Types of the relationships can be established on the basis of participant ETs

9. Relationships Types
A Binary relationship is the one that links two entity types e.g. STUDENT-CLASS

10. Binary Relationship Example
STD
COURSE
BOOK
EMP
PROJ

11. Binary Relationships
May also have instances, that can be formally described in an ordered pair form
{(S1001, OS), (S1020, DS), (S1002, DS), (S1058, NW)}

12. Attributes of the Rships
The key
The relationships can have their descriptive attributes
Where to place

13. Attributes of Rships
STD
COURSE
GRADE

14. Ternary Relationships
One that involves three entity types
STUDENT-CLASS-FACULTY

15. Ternary Relationship Examples

16. STD
COURSE
FACULTY
PROJ
EMP
SKILL

17. Ternary Relationships
Instances in ordered triples
Example {(S1013, MCS4, Adnan), (S1023, MCS3, Fasih)}

18. Unary Relationship
An ET linked with itself, also called recursive relationship
Example Roommate, where STUDENT is linked with STUDENT

19. Unary Relationship Examples
STD
ROOMMATE
EMP
CHAIR-PERSON
SPOUSE

20. Cardinality of Rships
Number of instances of one entity type that can possibly be related to instances of other entity type

21. Types of Cardinalities
One to one
One to many
Many to one
Many to many

22. Types of Cardinalities
DEPT
Chair
Person
One to one
EMP
DEPT
One to many/ many to one
PROJ
EMP
Many to many

23. Database Management System
Lecture - 9