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.