使用子查询进行查询

例如，仅为演示 HOW-TO 在 select 语句中使用子查询 select 语句，假设我们找到所有尚未编译地址的用户（地址表中不存在任何记录）：

 // get an ExpressionBuilder instance, so that you
$expr = $this->_em->getExpressionBuilder();

// create a subquery in order to take all address records for a specified user id
$sub = $this->_em->createQueryBuilder()
    ->select('a')
    ->from($this->_addressEntityName, 'a')
    ->where('a.user = u.id');

$qb = $this->_em->createQueryBuilder()
    ->select('u')
    ->from($this->_userEntityName, 'u')
    ->where($expr->not($expr->exists($sub->getDQL())));

return $qb->getQuery()->getResult();