Some quizzes coming from backup and restore randomly have 'ghost' questions that affect the grades. For instance an affected quiz is graded out of 12 but there are only 6 questions set to 1 grade
each. the bad data <question values =0> came from a
restored quiz where a teacher deleted some questions from the
quiz, then added others... so the deleted questions are the ones that
were retained in the instances table with a question value of 0 but
still having a grade of 1.

Use the following sql:

SELECT distinct(qq.quiz) as "quiz id", q.timeopen, q.timeclose, c.shortname
as "course shortname", c.id as "coursid" FROM "mdl_quiz_question_instances"
as qq
join mdl_quiz as q on q.id = qq.quiz
join mdl_course as c on c.id = q.course
WHERE "question" = '0'
order by q.timeclose