title: COUNT query on a boolean column with DQL
body: <p>I have a "simple" question whose answer seems not to be in the official DQL docs. 
  Given an application built with Doctrine2, suppose to have a <strong>boolean column</strong> (let say <em>B</em> for entity <em>Foo</em>), how to count the number of rows such that the value in B is true (or false)?</p>
  
  <p>PS: If may helps, I use MySql</p>