title: Difference between Primary key and Candidate key
body: <p>I have read about Keys in RDBMS. 
  <a href="https://stackoverflow.com/a/6951124/1647112">https://stackoverflow.com/a/6951124/1647112</a>
  I however couldn't understand the need to use a candidate key. If a primary key is all that is needed to uniquely identify a row in a table, why is candidate key required? 
  Please give a good example as to state the differences and importance of various keys.
  Thanks in advance. </p>