Some SQL engines support generating and evaluating queries. I stumbled upon a function in MariaDB code base that is introduced and keps specifically for that use case (it returns properly quoted SQL value as a string, including "NULL" string for null values). It is aptly named QUOTE [1].

[1] https://mariadb.com/kb/en/quote/

You can then specify columns as a query, group_concat the result and evaluate resulting statement [2].

https://mariadb.com/kb/en/execute-immediate/

I hope that helps.