Key integrity

Relations are defined as a unique set of tuples where no two tuples have the same combination of values for all their attributes. To be distinct, they must have some primary key that allows to reference each tuple. Key constraints deal with this issue.

Candidate key:

Each attribute or minimal combination of attributes that uniquely identifies any tuple in a relation is called a candidate key. Minimal means that removing an attribute leaves the key without the ability to uniquely identify any tuple and therefore not being a candidate key anymore.

Primary key:

The primary key is one chosen key candidate that acts as the identification key for a relation. Usually this is a short attribute like a ID-number (identification key) or username. Attributes of the primary are commonly underlined.