title: When to use CursorJoiner / MatrixCursor / MergeCursor?
body: <p>I'm exploring different ways to get data elegantly from two or more joined tables.</p>
  
  <p>I believe <code>MergeCursor</code>, (<a href="http://developer.android.com/reference/android/database/MergeCursor.html">Android Developer Guide</a>) seems to imply that could (for example) replace an equivalent <code>SQL UNION</code> by concatenating two queries (or adding views individually as rows, etc) - so, not what I want.</p>
  
  <p>But I'm at a loss as to what exactly <code>CursorJoiner</code> and <code>MatrixCursor</code> are for, or how to use them. I've looked at the source for them and (as usual) it means nothing to me! The examples I've found of them in use didn't clearly explain what the resulting effect was. I would really appreciate a good description of them, and the context they might be used in. </p>