title: INSERT into a table from SELECT only if value doesn't exist
body: <p>I want to insert <code>table1.id</code> into <code>table2.t1col</code> only if <code>table1.id</code> doesn't exist in <code>table2.t1col</code>, yet.</p>
  
  <p>I think I have to use :</p>
  
  <pre><code>insert into table2 name (t1col) value (select id from table1)
  </code></pre>
  
  <p>but I want to add only if that <code>id</code> doesn't exist in <code>table2</code> already.</p>