Tim Hunt

Which Moodle version is this?

In recent versions, on the Quiz -> Questions page (e.g. https://qa.moodledemo.net/mod/quiz/edit.php?cmid=1171) you can see each question is badged with a clickable link to the question bank where it comes from.

If something was fixed in 5.0.3 then that fix will also have been added to 5.1. If you need to verify that, you can with a git command like

git log --grep MDL-85869 upstream/MOODLE_501_STABLE

(If you are familiar with developer tools like git.)

As Visvanath says, to understand the problem, we need more information about the exact error you are getting and/or the data that is leading to it.

Hidden status exists because, when people are tidying up or re-organsing their question banks, we don't want past quiz attempts or existing quizzes to break.

So, if you try to delete a question that is needed by something, then we don't acutally delete the data from the database. Instead we just set the state to hidden. Then, it is no longer visible (unless you turn on the option to show hidden questions, which is there in case you need to see what is really going on.)

Does that make sense?

Not strictly true. They lead to an index or unique index being created, which is good for query performance. But, as you say, they don't enforce the foreign-key constraint.

That is not how you find the questions in a categroy. question.parent is used by Cloze - it is not how questions are linked to categories.

You need to look at question_bank_entries.questioncategoryid.

As part of the upgrade from Moodle 4.x to Moodle 5, there was some some code that automatically fixed up that problem as best as possible, before doing the main upgrade. So, doing what that code does would be the best thing: https://github.com/moodle/moodle/blob/52c6473e160c1985d95d6e5a5f44c1678ef2b2fe/public/mod/qbank/classes/task/transfer_question_categories.php#L280