18. In a relational database table, a foreign key column can contain: (a) NULLS

Question

18. In a relational database table, a foreign key column can contain: (a) NULLS or blanks (b) duplicate values (c) both a and b above (d) neither a nor b above 19. A small college database contains INSTRUCTOR and CLASS entities/tables. Instructor-ID; PK of CLASS table is Class-Code. To create a relationship between these two tables, what would PK of INSTRUCTOR table is you do? (a) include Instructor-ID column in CLASS table as a foreign key column (b) include Class-Code in INSTRUCTOR table as a foreign key column (c) place a third table between INSTRUCTOR and CLASS tables 20. In ER diagrams, the term "many" means: (a) any number zero or more (c) any number including zero, less than zero, and more than zero (b) any number one or more

Explanation / Answer

18. Answer is option c.
Foreign key column can have duplicates or nulls. Foreign Key requires that the value first exists in the parent table. A null would just mean that the value is not determined yet.

19. Answer is option a.
This is because a class can be handled only by a single instructor, whereas an instructor can take up many classse. Hence including instructor id in class table seems to be more appropriate.

20. Answer is option b.

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