为什么列别名在学说中不起作用？

My script is like this:

我的脚本是这样的:

$query = Doctrine_Query::create ()
  ->select('count(p.product_id) as num_a')              
  ->from ( 'ProductComments p' )
  ->groupBy('p.product_id')
  ->having('num_a =2 ');
$query =