5.19 Give examples of binary 1:N relationships, other than those in this text, f

Question

5.19 Give examples of binary 1:N relationships, other than those in this text, for (a) an

optional-to-optional relationship, (b) an optional-to-mandatory relationship, (c) a

mandatory-to-optional relationship, and (d) a mandatory-to-mandatory relation-

ship. Illustrate your answer by using IE Crow’s Foot E-R diagrams.

5.21 Explain the meaning of the term intersection table.

5.23 For your answers to questions 5.20, 5.21, and 5.22, describe a method for obtaining

the children for one of the entities in the original data model, given the primary key

of the table based on the second entity. Also, describe a method for obtaining the

children for the second entity, given the primary key of the table based on the first

entity.

5.25 Why is it not possible to represent N:M relationships with the same strategy used

to represent 1:N relationships?

5.27 Give an example of a 1:N relationship with an ID-dependent weak entity, other

than one shown in this text. Illustrate your answer using an IE Crow’s Foot E-R

diagram.

5.29 Define the three types of recursive binary relationships and give an example of

each, other than the ones shown in this text.

Explanation / Answer

Please find my answer for Q21, 23, 25

Please repost others in separate post.

21. Explain the meaning of the term intersection table .
Ans: The term intersection table is when a new relation is made from the keys for each of the tables that formed it.

22. Explain how the terms parent table and child table relate to the tables in your answer to question 5.20.
Ans: The student table is the parent table and the classes table is the child table. Given the StudentID we can get the student’s schedule and the class descriptions.

23. For you answer to question 5.20, 5.21, and 5.22, describe a method for obtaining the children for one of the entities in the original data model, given the primary key of the table based on the second entity. Also, describe a method for obtaining the children for the second entity, given the primary key of the table based on the first entity.
Ans: Given the StudentID, we can look up the appropriate row in Student-Class and the Schedule of that Student. Since we obtained the schedule, we can look up the ClassDesc that our student has in his schedule.

25. Why is it not possible to represent N:M relationships with the same strategy used to represent 1:N relationships?
Ans: Because in a 1:N relationship we place the foreign key from one table into the other and in the N:M relationship we create a new relation called an intersection relation.

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