Question

Cardinality ratios often dictate the detailed design of a database. The cardinality ratio depends on the real-world meaning of the entity types involved and is defined by the specific application. For the binary relationships below, suggest cardinality ratios based on the common-sense meaning of the entity types. Clearly state any assumptions you make.

Entity 1 Cardinality Ratio Entity 2
1. Student SocialSecurityCard
2. Student Teacher
3. ClassRoom Wall
4. Country CurrentPresident
5. Course TextBook
6. Item (that can
be found in an
order) Order
7. Student Class
8. Class Instructor
9. Instructor Office
10. E-bay Auction item E-bay bid

Answer 1

ENTITY 1 CARDINALITY RATIO ENTITY 2

1. Student 1 to many Social security card

A student may have more than one

social security card (legally with the

same unique social security number),

and every social security number belongs

to a unique.

2. Student Many to Many Teacher

Generally students are taught by many

teachers and a teacher teaches many students.

3.ClassRoom Many to Many Wall

Do not forget that the wall is usually

shared by adjacent rooms.

4. Country 1 to 1 Current President

Assuming a normal country under normal

circumstances having one president at a

time.

5. Course Many to Many TextBook

A course may have many textbooks and

text book may be prescribed for different

courses.

6. Item Many to Many Order

Assuming the same item can appear

in different orders.

7. Student Many to Many Class

One student may take several classes.

Every class usually has several students.

8. Class Many to 1 Instructor

Assuming that every class has a unique

instructor. In case instructors were allowed

to team teach, this will be many-many.

9. Instructor 1 to 1 Office

Assuming every instructor has only one

office and it is not shared. In case of offices

shared by 2 instructors, the relationship

will be 2-1. Conversely, if any instructor has a joint

appointment and offices in both departments,

then the relationship will be 1-2. In a very general

case, it may be many-many.

10. E-bay Auction item 1-Many E-bay bid

1 item has many bids and a bid is unique

to an item.