The following answers were NOT correct:
Candidate Key: A candidate key is a combination of attributes that can be uniquely used to identify
a database record without any extraneous data. Each table may have one or more candidate
keys. One of these candidate keys is selected as the table primary key.
Foreign Key: A foreign key is a field in a relational table that matches the primary key column of
another table. The foreign key can be used to cross-reference tables.
Secondary key: The term secondary key is a key that is used strictly for data-retrieval purposes. A
secondary key is sometimes defined as a "data item value that identifies a set of records." It is
important to note that a secondary key does not need to have unique values in a table; in this
respect, secondary keys differ from primary keys (and candidate keys and superkeys).
References:
A candidate key is an attribute that is a unique identifier within a given table.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten
Domains of Computer Security, 2001, John Wiley & Sons, Page 45.
Candidate Key Ref: http://databases.about.com/cs/specificproducts/g/candidate.htm
Feedback from Jerry: A candidate key is one of several alternative columns in a table that may be
chosen as a primary key. The PRIMARY KEY IS the unique identifier. The foundation of a
relational database is the establishiment and reliance on a unique primary key, not candidate
keys. Primary key is a more correct answer to this question than candidate key.
Secondary key ref: http://www.gslis.utexas.edu/~wyllys/DMPAMaterials/keys.html