In one of my controllers I have:

function TipsByCategory()
   {
      if(isset($_GET['id']))
      {
         $tips = DataObject::get('Tip', "tip_categories.CategoryID = $_GET[id]", '', "LEFT JOIN tip_categories ON tip.ID = tip_categories.TipID");
         return $tips;
      }
   }

But this doesn't seem to be working in the 'live' site. I figured out by the errors and looking in the database that for the 'live' site I had to use

ON tip_live.ID = tip_categories.TipID

instead of

ON tip.ID = tip_categories.TipID

Now I'm probably doing this the wrong way... any ideas?

No, doesn't seem to do it here on my installation 2.1.0 rc1

Ah, are you using Windows?

There's a problem with MySQL on Windows - by default it doesn't care about the case-sensitivity of table names, but Linux does. This is because Windows itself is a case-insensitive file system, whereas Linux isn't.

If you want to fix that, there's an option you can add to your my.ini file for MySQL, take a look at the MySQL docs here which explain it in more depth. If I ever build a Windows server, I use lower_case_table_names = 2, which preserves the original case - e.g. it will create the table 'Tip' instead of 'tip'. This means your code will port fine to Linux later, and you can just copy the database over without issues.