Database System Concepts
7th Edition
ISBN: 9780078022159
Author: Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher: McGraw-Hill Education
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Expert Solution & Answer
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
- 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.
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