Prøv denne :
$psub=$this->db->select()
->setIntegrityCheck(false)
->from(array('p'=>'projecttouser'),array('count(p.u_id) as count'))
//->join(array('i'=>'usermaster'),'p.u_id=i.id') // no need for this join
->where('usermaster.id=projecttouser.u_id');
$tsub=$this->db->select()
->setIntegrityCheck(false)
->from(array('t'=>'tasktotarget'),array('count(t.assigned_to) as tcount'))
->where('usermaster.id=tasktotarget.assigned_to');
$tasub=$this->db->select()
->setIntegrityCheck(false)
->from(array('ta'=>'tasktotarget'),array('count(ta.assigned_to) as tacount'))
->where('usermaster.id=tasktotarget.assigned_to AND tasktotarget.is_active = 1 ');
$sql=$this->db->select()
->setIntegrityCheck(false)
->from(array('u'=>'usermaster'), array('usermaster.*',
'proj' => new Zend_Db_Expr('(' . $psub . ')'),
'target' => new Zend_Db_Expr('(' . $tsub . ')'),
'active' => new Zend_Db_Expr('(' . $tasub . ')')))
->group('u.id')
//->order($order_by . ' ' . $order)
->where('u.is_delete=false');
$resultSet = $this->fetchAll($sql);
return $resultSet;
Doc :http://framework.zend.com/manual /1.12/da/zend.db.select.html