title: Why use primary keys?
body: <p>What are primary keys used aside from identifying a unique column in a table?  Couldn't this be done by simply using an autoincrement constraint on a column?  I understand that PK and FK are used to relate different tables, but can't this be done by just using join? </p>
  
  <p>Basically what is the database doing to improve performance when joining using primary keys?</p>