title: What is the difference b/w Primary Key and Unique Key
body: <p>I tried to find it out in google but not satisfactory answer is given out there. Can anybody explain the solid difference.   </p>
  
  <p>actually if Primary key is used to select data uniquely then what is the need of Unique key?  </p>
  
  <p>When should I use a Primary key and when to use a Unique key?</p>