title: mysql conditional insert
body: <p>can we have sql insert like this?</p>
  
  <p><code>
  INSERT INTO <code>A_has_B</code> (<code>A_id</code>,<code>B_id</code>) VALUES (1,2) IF NOT (SELECT COUNT(<code>A_id</code>) FROM <code>A_has_B</code> WHERE <code>A_id</code> = 1 AND <code>B_id</code> = 2);
  </code></p>