System information

About database basics 33
About relational tables
In a database, you can organize data in multiple tables. For example, if you manage a database for
the Human Resources department, you might have one table that lists all the information about
employees and another table that lists all the departments:
You have multiple departments for employees, but you would not store the information about the
departments in every employee row for the following reasons:
• The department information is the same for each employee in a given department; however,
repeating the department information for each employee is redundant. Storing redundant data
takes up more disk space.
• If the department information changes, you can update one occurrence. All references to that
department are updated automatically.
Storing multiple occurrences of the same data is rarely a good thing. Good relational database
design separates application entities into their own tables. Key values from one table are often
stored in a related table rather than repeating the information. The key value is used to join the
data between the tables to return the complete set of data required.