1. Database Systems
Instructor Name:
Lecture-8

2. Contents
Relationship
Attribute on Relationship
Associative Entity 2

3. Relationship
An relationship is an association that represent the interaction among Entity Instances of one or more Entity Types that is of interest to the organization
It is Verb or Verb Phrase 3

4. Relationship Type
An relationship Type is an Abstraction of Meaningful Associations among Entity Types
It can answer the questions that is of interest to the organizations and these Answers cannot be answered by only using Entity Types 4

5. Relationship Instance
It is an association between/ among entity instances where each relationship instance associates with exactly one entity instance from participating Entity Types
A relationship instance in an E-R schema represents an association between the named entities in the real-world enterprise that is being modeled. 5

6. Relationship Set
It is a set of associations or relationships among entities from participating Entity Types
Formally, it is a mathematical relation on n ≥ 2 (possibly nondistinct) entity sets. If E1, E2, , En are entity sets, then a relationship set R is a subset of {(e1, e2, . . ., en) | e1 ∈ E1, e2 ∈ E2, , en ∈ En}
Alternatively, it can be defined as a subset of the Cartesian product of the entity sets E1 ×E2 × ... × En 6

7. Relationship Set-Contd.
if A, B are sets, then a relation R is a subset of A  B
A={1,2,3}, B={a,b,c,d},
A  B = {(1,a),(1,b), . . ., (3,d)}
R = {(1,a), (1,c), (3,b)}
- makes is a subset of Product  Company: 1 2 3 a b c d A= B=
makes
Company
Product 7

8. Participation and Role
The association between Entity Sets is called Participation that is the Entities E1,E2,E3,…,En participate in a relationship R.
The function that an entity plays in a relationship is called that entity’s role.
It defined whether an Entity Instance of one Entity Type can exist without participating with an Entity Instance of another Entity Type.
The Role is implicit unless there is a Recursive Relationship 8

9. Relationship – Participation9

10. Relationship - Role 10

11. Example – Relationship Type, Relationship Instance and Relationship Set11

12. Relationship Instances12

13. Attribute on Relationship
It is a relationship that is dealt as an attribute in some situations. In other words, attributes may be related with relationships
Where we can place the attribute Date Complete?(next slide)
There is a value of the date for each instance of relationship
Important
Attribute on Relationship only occur in many-to-many relationship
They do not occur in one-to-one or one-to-many relationship 13

14. Attribute on a Relationship – Contd.14

15. Attribute on a Relationship- Contd.15

16. Attribute on a Relationship- Another Example
A student is assigned to a supervisor on a specific date 16

17. Associative Entity
A solution to attribute on relationship
Associative entity An entity type that associates the instances of one or more entity types and contains attributes that are peculiar to the relationship between those entity instances.
Entity Type-Noun Relationship - Verb 17

18. Associative Entity – Defining Rules
The participating entity types are in “many” relationships
Resulting Associative Entity has independent meaning to the end user
The associative entity has one ore more attribute along-with Identifier Attribute
The associated entity participate in one or more relationship independent of the entities associated in the relationship 18

19. Assignment – What I was Expecting
major
Department
offers
Is double major allowed?
Can a student have more than 1 advisor?
Is joint appointment of faculty possible?
Can two profs share to teach the same course?
Can a professor teach more than one course?
Can a professor stay without affiliated with a department?
faculty
Courses
teaches
Professor
advisor
enrollment
Students 19

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