Oracle PL/SQL Tutorial/Introduction/Terminology

Understanding basic database terminology

A database consists of tables and columns.

Database Design Terminology

Logical/Relational Logical/Object-Oriented Physical Implementation Entity Class Table Attribute Attribute Column Instance Object Row

Entity: An entity maps something in the real world. For example, departments within an organization, employees, or sales.

Attribute: Represent information about an entity instance or an object. For example, the birth date or Social Security number of an employee.

Entities (classes) are implemented in the database as tables.

Attributes are implemented in the database as columns.

Instances (objects) are implemented in the database as rows.

A primary key uniquely identifies a specific instance of an entity.

No two instances of an entity can have the same primary key.

The values of all parts of the primary key must never be null.

The most common types of primary keys in relational databases are ID numbers.

Sometimes more than one attribute (or sets of attributes) can be used as a primary key.