Expert: Ivan Maskovic Date: 8/16/2007 Subject: FUNDAMENTALS OF DATABASE
Question Sir,
i want to know the definitions of a key,primary key,secondary key & the difference among these.
if someone asks me what is the difference between Database management system & Relational Database management system then what should be my answer?
to shorten it:
superkey (better term than key) is set of attributes of a relation (table) for which it holds that there are no two distinct tuples (rows) that have the same values for the attributes in this set
candidate key is minimal (irreducible) superkey
(note that relation can have more than 1 candidate key)
primary key is arbitrally chosen amongst candidate keys
secondary(alternate) key is candidate key not chosen as primary
As for DBMS - RDBMS
DBMS - DataBase Manegement System - Set of software purpose of which is to manage database - storage, manipulation, query language to name just few
RDBMS - DBMS based on Relational Database Model (invented by E.F.Codd) - information is represented as relations
Commercial RDBMS-es are not true RDBMS-es better term would be SQL DBMS.
