Add select COUNT events in common between the current user and the others in DQL query - postgresql

Question

I have this DQL query in Symfony 4 project, it does select all users who are registered in all the same events as current user
$query = $this->_em->createQuery(
"select u as user, er.dateRegistration from App:EventRegistration er
left join App:User u with er.user = u.id
where er.user != :user_id and
er.event in (select identity(er2.event) from App:EventRegistration er2
where er2.user = :user_id)")
->setParameter('user_id', $user->getId());
return $query->getResult();
Now, in the same query, I'd like to select COUNT how many times each one of the other users has registered in the same event of the current user.
I mean the number of events in common between the current user and the others.
if I add count(er.id) to the select
select u as user, er.dateRegistration, count(er.id)
I get this error :
SQLSTATE[42803]: Grouping error: 7 ERREUR: la colonne « u0_.username » doit apparaître dans la clause GROUP BY ou doit être utilisé dans une fonction d'agrégat
LINE 1: SELECT u0_.username AS username_0, u0_.username_canonical AS..

Answer 1

You need to add group by;
$query = $this->_em->createQuery(
"select u as user, count(er.id) as register_count from App:EventRegistration er
left join App:User u with er.user = u.id
where er.user != :user_id and
er.event in (select identity(er2.event) from App:EventRegistration er2
where er2.user = :user_id) group by u.id")
->setParameter('user_id', $user->getId());
return $query->getResult();
It will give you register count of each user but in this case you can not expose the "er.dateRegistration" because now you are grouping counts for every user.