Chapter 2, Problem 1PE

Explanation of Solution

Primary key of a relation:

• A primary key is a special column of a relation used for uniquely identifying a relation.
• The primary key can be one column or combination of columns. The primary key will uniquely identify all the records of a table.
• The features of a primary are:
  • The primary key contains a unique value for each row of data.
  • The primary key column cannot contain null values.

Identifying the primary key in the given database:

• The employee database is given below,

employee (person_name, street, city)

works (person_name, company_name, salary)

company (company_name, city)

• The database has three relations that are, employee, works, and company.
• The primary key for the first relation can be “person_name” that can identify the employees in the relation. The other available columns are “street” and “city” and that can come common to many employees. So the columns “street” and “city” cannot identify the employees in the table.
• The primary key for the second relation can be “person_name” that can identify the employees in the relation. The other available columns are “company_name” and “salary” and that can come common to many works. That is may companies provide same kind of jobs with same amount of salary. So the columns “company_name” and “salary” cannot identify the rows in the table.
• The primary key for the third relation can be “company_name” that can identify the company in the relation. The other available column is “city” and that can come common to many companes. So the column “city” cannot identify the companies in the table.
• The relations with primary key are given below,

employee (person_name, street, city)

works (person_name, company_name, salary)

company (company_name, city)

• The primary keys are represented as underlines columns.