select * from questions
left join responses on responses.QuestionId = questions.id
where responses.username <> 'username string' or responses.username is null
order by rand() limit 1;

_sequelize.query(
'select * from questions ' +
'left join responses on responses.QuestionId = questions.id ' +
'where responses.username <> "' + req.query.name + '" or responses.username  is null ' +
'order by rand() ' +
'limit 1',
{model : models.Responses})