Chapter 3, Problem 18P

Program Plan Intro

Primary Key:

A Primary Key in a database table is a field in the table that uniquely identifies every row or record present in the database table.

Example:

Students in Universities are assigned a unique registration number.

Therefore, in a STUDENT database table, the attribute “reg_no” acts as primary key.

Foreign Key:

Foreign Key is a column in a relational database table which provides a relation between two tables. It provides a cross reference between tables by pointing to primary key of another table.

Example:

In STUDENT database table, the attribute “reg_no” acts as primary key and in COURSE database table in which the student selects his or her course, the same “reg_no” acts as foreign key for the STUDENT table.

Entity Integrity:

Entity Integrity is a mechanism that is used to protect primary keys in the database. Entity Integrity ensures the following properties for primary key:

• The primary key of one database table row must not match the primary key of any other row present in the table.
• No component of primary key must be null.