title: mongoDB vs mySQL -- why one is better than another in some aspects
body: <p>I am really new to database and am interested in some high level basic knowledge. I have read <a href="https://stackoverflow.com/questions/1476295/when-to-use-mongodb-or-other-document-oriented-database-systems">this wonderful SO post</a>. I under one is better than another in some cases, but not sure why. </p>
  
  <ol>
  <li><p>Why is MySQL faster than MongoDB at join operations? </p></li>
  <li><p>Why does MongoDB scale better in distributed system? </p></li>
  <li><p>Why is MongoDB faster if I am "just selecting a bunch of tables and putting all the objects together, AKA what most people do in a web app" ? </p></li>
  </ol>
  
  <p>Thanks a lot!</p>