Key Types

Key Type	Definition
Superkey	An attribute (or combination of attributes) that uniquely identifies each row in a table
Candidate key	A minimal superkey. A superkey that does not contain a subset of attributes that is itself a superkey
Primary key	A candidate key selected to uniquely identify all other attribute values in any given row. Cannot contain null entries.
Secondary key	An attribute(or combination of attributes) used strictly for data retrieval purposes.
Foreign key	An attribute(or combination of attributes) in one table whose value must either match the primary key in another table or be null.