Vous avez deux options
INNER JOIN x
ON x.qid = y.qid OR (x.qid IS NULL AND y.qid IS NULL)
ou plus simple
INNER JOIN x
ON x.qid IS NOT DISTINCT FROM y.qid
Vous avez deux options
INNER JOIN x
ON x.qid = y.qid OR (x.qid IS NULL AND y.qid IS NULL)
ou plus simple
INNER JOIN x
ON x.qid IS NOT DISTINCT FROM y.qid