Potential key

A candidate key (in the relational data model) is a subset of relationship attributes that meets the requirements of uniqueness and minimality ( irreducibility ).

• Uniqueness means that there are no and cannot be two tuples of a given relation in which the values ​​of this subset of attributes coincide (are equal).
  The uniqueness property is determined not for the specific value of the variable of the relationship at any given time, but for all possible values, that is, follows from the external knowledge about the nature and patterns of data that can be in the variable of the relationship.
• Minimality (irreducibility) means that the potential key does not contain a smaller subset of attributes satisfying the uniqueness condition. In other words, if you remove any attribute from the potential key, it will lose its uniqueness property.

Since all tuples in a relation are, by definition, unique, there always exists at least one potential key in it (for example, including all attributes of the relation).

In relation there can be several potential keys simultaneously. One of them can be selected as the primary key of the relationship , then the other potential keys are called alternative keys .

Theoretically, all potential keys are equally suitable as a primary key; in practice, one of the potential keys that has a smaller size (physical storage) and / or includes fewer attributes is usually selected as the primary key.