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Practice Exercises
2.1 Consider the employee database in below Figure. What are the appropriate primary keys?

Primary key is an attribute in a table which can uniquely identify that table. In employee table the person_name, in works table person_name and in company table company_name are the primary keys.

2.2 Consider the foreign-key constraint from the dept name attribute of instructor to the department relation. Give examples of inserts and deletes to these relations that can cause a violation of the foreign-key constraint.

Insertion:
As Instructor is child table of department for attribute dept_name, then adding something in instructor table which is not in department table will cause error. For example if we add economics as dept_name in instructor table but department table don’t contain it then it is a violation of foreign key constraint.

Deletion:
As department is parent table of instructor for attribute dept_name, then deleting something in department table which is present in instructor table will cause error. For example if we delete economics as dept_name in department table but instructor table do contain it then it is a violation of foreign key constraint.

2.3 Consider the time slot relation. Given that a particular time slot can meet more than once in a week, explain why day and start time are part of the primary key of this relation, while end time is not.

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