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

Solution :

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.