title: How to join three table
body: <p>I'm having three tables, two of which are same in structure</p>
  
  <p><strong>table1 like this</strong></p>
  
  <pre><code>-----------------------------------------------
  id       title     description       image
  -----------------------------------------------
  1         title1   desc1             image1.png
  2         title2    desc2            image2.jpg
  ------------------------------------------------
  </code></pre>
  
  <p><strong>table2 like this</strong></p>
  
  <pre><code>-----------------------------------------------
  id       title     description       image
  -----------------------------------------------
  1         ttl1   des1             img1.png
  2         ttl2    des2            img2.jpg
  ------------------------------------------------
  </code></pre>
  
  <p><strong>table 3 like this</strong></p>
  
  <pre><code>-----------------------------------------------
  id       table1_id    table2_id     
  -----------------------------------------------
  1         0             1             
  2         1             0    
  ------------------------------------------------
  </code></pre>
  
  <p>I want to join table3 to one of the other tables where id is not zero.  </p>