Integrity constraints are necessary because data in a database must be valid and consistent at all times. Data might be inconsistent because of entry errors, duplicate entries of rows, or other violations.
For example, to properly track a company's employee information, each employee should be assigned a unique identification number. To ensure this, specify a UNIQUE constraint on the column that contains the employee number (emp_no), as shown in the following example.
CREATE TABLE employee_info (
emp_no INTEGER NOT NULL UNIQUE,
first_name VARCHAR(20) NOT NULL,
last_name VARCHAR(20) NOT NULL,
title VARCHAR(20)
) ;
