6A:Artificial, Composite, and Secondary UID

Purpose

The unique identifier is the value or combination of values that enables the user to find that one unique item among all the rest. Identifying the right attribute, combination of attributes, and/or relationships is a skill that any database designer must master.

Artificial UIDs

Artificial UIDs are those that don't occur in the natural world but are created for the purposes of identification in a system. For example, people are not born with "numbers," but many systems assign unique numbers to people in order to identify people. 

Candidate UIDs

Sometimes there is more than one possible UID. For example, when you order a product from a commercial website, you will usually be assigned a unique customer code and also be asked to enter your e-mail address. Each of these uniquely identifies you, and therefore each could be used as a UID. They are both candidate UIDs. 

6C:Second Normal Form

• Second Normal form requires that any non-UID attribute be dependent on the entire UID.

6B:Normalization and First Normal Form

• First Normal form requires that there be no multivalued attributes
• To check for 1NF, validate that each attribute has a single value for each instance of the entity.
• If an  attribute has repeating values, create an additional entity and relate it to the original entity with a M:1 relationship

6D:Third Normal Form

• The rule of Third Normal Form states that no non-UID attribute can be dependent on another non-UID attribute.
• Third Normal Form prohibits transitive dependencies. A transitive dependency exists when any attribute in an entity is dependent on any other non-UID attribute in that entity.

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.