Sometimes questions apply to multiple categories. For example, suppose there's a Quiz about Moodle development. There are some categories such as:
Moodle Development
Moodle Activities
SCORM
Quiz
Moodle Internals
Coding standards
Database
There is a question which asks "name two of the database tables used by the Quiz activity". That question is properly linked to the category "Moodle Activities > Quiz" AND in the category "Moodle Internals > Database".
They say a picture is worth a thousand words, so here's an illustration:
Has any thought been given to adjusting the DB structure to allow this to be represented?
Reading the use cases for question "tags", it would seem that for many use cases, allowing a question to be linked to multiple categories would be a better method of meeting the need. Tags, as proposed, can't be structured, so the tag could only be "database", not "Moodle Internals > Database". So this aproach might give better results for users. Additionally, Quiz already allows one to select X random questions from a category, for example, so implmenting this functionallity via categories would use existing functionality, with the existing UI, rather than cluttering up the UI and the code with essentially duplicate functions calling them "tags' rather than "categories".
That applies of course to existing code in the master download, and also to extras like the report at https://moodle.org/mod/forum/discuss.php?d=219932 which is a report showing grades per category. I don't know if there's anything to be gained by creating a need to copy and paste the same code, changing the word "category" to the word "tag". It seems to me allowing for multiple categories fills the need, plus some.