An attribute that uniquely defines an entity in an entity set. They can be super, candidate or primary.
\!h Super key
A set of attributes that together define an entity in an entity set.
\!h Candidate key
A minimal super key; has the least possible number of attributes to still be a super key.
An entity set may have more than one candidate key.
\!h Primary key
A candidate key chosen to uniquely identify the entity set.
\!h Foreign key
Identifies the relationship between entities.