如何使用 Doctrine 2 DBAL 连接子查询？

$subSelect = $db->select()
    ->from('user_survey_status_entries', array('userSurveyID', 'timestamp' => 'MIN(timestamp)'))
    ->where('status = ?', UserSurveyStatus::ACCESSED)
    ->group('userSurveyID');


$select = $db->select()
    // $selectColNames contains columns both from the main query and 
    // the subquery (e.g. firstAccess.timestamp AS dateFirstAccess).
    ->from(array('us' => 'user_surveys'), $selectColNames)
    ->joinLeft(array('firstAccess' => $subSelect), 'us.userSurveyID = firstAccess.userSurveyID', array())
    ->where('us.surveyID = ?', $surveyID);

$subSelect = $connection->createQueryBuilder()
    ->select(array('userSurveyID', 'MIN(timestamp) timestamp'))
    ->from('user_survey_status_entries')
    // Instead of setting the parameter in the main query below, it could be quoted here:
    // ->where('status = ' . $connection->quote(UserSurveyStatus::ACCESSED))
    ->where('status = :status')
    ->groupBy('userSurveyID');

$select = $connection->createQueryBuilder()
    ->select($selectColNames)
    ->from('user_surveys', 'us')
    // Get raw subquery SQL and wrap in brackets.
    ->leftJoin('us', sprintf('(%s)', $subSelect->getSQL()), 'firstAccess', 'us.userSurveyID = firstAccess.userSurveyID')
    // Parameter used in subquery must be set in main query.
    ->setParameter('status', UserSurveyStatus::ACCESSED)
    ->where('us.surveyID = :surveyID')->setParameter('surveyID', $surveyID);