2B:Entities, Instances, Attributes, and Identifiers

Purpose: Knowing how to organize and classify data makes it possible to draw useful conclusions about seemingly random facts. Our technology-rich world produces vast quantities of facts in need of structure and order. 


 - It is important to learn about these entities because they are the things abut which we store data.
 - It is important to learn about attributes because they provide more information about entities. 
 - It is important to learn about unique identifiers because they distinguish one instance of an entity from another.

An Entity is...

- Something of significance to a businesss about which data must be known.
- A name for a set of familiar things you can list.
- Usually a noun.

Examples:Objects, events, people.

*Remember* : Entities have instances. An instance is a single occurrence of an entity.

What is an Attribute?

 - Like an entity, an attribute represents something of significance to the business.

An attribute is a specific piece of information that:

• Describes an entity.
• Quantifies an entity.
• Qualifies an entity.
• Classifies an entity.
• Specifies an entity.

 - Attributes are single-valued. Each attribute can have only one value (at any point in time) for each instance of the entity.

 - Some attributes, like "age", have values that are constantly changing. these types of attributes are referred to as volatile attributes.

 - Other attributes like "order date" will change rarely, if ever. These are referred to as nonvolatile attributes.
If there is ever a choice between attributes, use the nonvolatile one. For example: Use birth date instead of age.