i would say very similar to a test generator from an existing database, not data generator, but need to do it in App Inventor
Selection of random items from multiple lists is very straightforward.
I reiterate,
Not everything that is calculated needs to be stored.
Not everything that is stored need to be selected.
Not everything that is selected needs to be displayed.
Not everything that is displayed needs to be read.
.
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Selection is not much of an issue, I can inquire the mysql db with certain conditions to retrieve entries. The tricky part is to display them with dynamic components (particularly radiobuttons)
A small sliding window into a long list can be simple and convenient.
You probably encounter them online.
From my sample collection ...
https://groups.google.com/g/mitappinventortest/c/WEat3QJXUX4/m/-r3mNVSIAwAJ
Radio buttons:
randoms in range ...
https://groups.google.com/d/msg/mitappinventortest/pOWEZtYjx7s/mh0UrEJ5AQAJ
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Sounds a good idea to replace radiobuttons with listpicker for choices, now the cumbersome part is to display components dynamiclly. Not complete solution yet, but give you a 
you also might want to take a look at
A Multiple Choice Quiz: How to work with the advanced features
what is missing in the example is to read the questions and answers from the database...
Taifun
Aside from your firm belief that every photo needs a frame for it, there is the question of how to track the questions and answer choices in your database.
Your app sounds like your own version of Google Forms, which allows users to define their own forms and let other users enter data into those forms that would end up as rows in that Form's Google Sheet.
A Google Form is a form of schema, which you can represent in a dictionary.
also the components are not created dynamiclly in the example.
what is that? also i don't think what i am trying to make is sth very similar to google form
Will Mit punya's survey component do the work? 
Well, it is always one question and 4 possible answers to choose from...
Why do you think, you need something dynamic for that?
Taifun
the number of test questions are not certain because as long as the questions in db meet certain criteria, all questions should be retrieved. Also the number of choices for each question are not always the same (this is intentional, sorry that this is not reflected in my question, cannot edit it anymore).
can you share a link please?
Even if the block will generate test questions as i hoped, but how can i respond to click of a submit button for a specific question?
apparently, Im not alone here, see this post by Ayun_Challymalang
(https://community-appinventor-mit-edu.ezproxy.canberra.edu.au/u/Ayun_Challymalang)
Click event on a Button created with DynamicComponents
There's no solution for that post yet. Hope some smart cookie can help us here? Thanks
the example is able to process as much as questions and corresponding answers as you like...
you can load the question list with 3 questions or 100 questions, it does not matter for the logic used in the example...
you have to adjust the example to your needs... as already said
let's assume, there is a max. of 4 possible answers... if a question only offers 3 answers, then just set the 4th answer to visible = false
too complicated for a simple problem like a quiz...
you can have a fix question/answers layout... no need for a dynamic layout...
Taifun
see here:
but as said by @Taifun, you probably do not need to use dynamic components for what you want to do.
my determination to have dynamic component starts to shake, I will try to use a static layout. Do you see anything major out of place in the block i posted? Thanks.
Thanks for the information, it's great to know that. Will try.