How to Join Subqueries in Doctrine DBAL?

Join Subquery with Doctrine DBAL

In the process of refactoring a Zend Framework application to use Doctrine 2.5 DBAL, it can be challenging to translate complex queries from the previous Zend_DB format. One such challenge lies in joining subqueries, which were previously accomplished using methods such as joinLeft().

Although Doctrine DBAL does not natively support joining subqueries, there is a workaround that utilizes the raw SQL of the subquery. By wrapping the subquery SQL in brackets and using the sprintf() function, it can be joined as a regular table.

Example

Consider the following Zend_Db query:

// Subquery to find the minimum timestamp for each user survey.
$subSelect = $db->select()
   ->from('user_survey_status_entries', array('userSurveyID', 'timestamp' => 'MIN(timestamp)'))
   ->where('status = ?', UserSurveyStatus::ACCESSED)
   ->group('userSurveyID');

// Main query to join user surveys and subquery results.
$select = $db->select()
   ->from(array('us' => 'user_surveys'), $selectColNames)
   ->joinLeft(array('firstAccess' => $subSelect), 'us.userSurveyID = firstAccess.userSurveyID', array())
   ->where('us.surveyID = ?', $surveyID);

Doctrine DBAL Translation

In Doctrine DBAL, the raw SQL of the subquery is obtained as follows:

$subSelect = $connection->createQueryBuilder()
   ->select(array('userSurveyID', 'MIN(timestamp) timestamp'))
   ->from('user_survey_status_entries')
   ->where('status = :status')
   ->groupBy('userSurveyID');

$subSelectSQL = $subSelect->getSQL();

The subquery SQL is then wrapped in brackets and joined in the main query:

$select = $connection->createQueryBuilder()
   ->select($selectColNames)
   ->from('user_surveys', 'us')
   ->leftJoin('us', sprintf('(%s)', $subSelectSQL), 'firstAccess', 'us.userSurveyID = firstAccess.userSurveyID')
   ->setParameter('status', UserSurveyStatus::ACCESSED)
   ->where('us.surveyID = :surveyID')
   ->setParameter('surveyID', $surveyID);

This approach allows for joining subqueries in Doctrine DBAL, while maintaining the ability to dynamically extend the query later in the code.

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