There already are answered questions on how to generate unique, random alphanumeric strings in MySQL. That is not the question.
I am trying to generate a 4 character, random alphanumeric string in MySQL such that the COMBINATION of that field and another are unique in MySQL. In other words, given an existing field in the table (userID), I need a 4 character, random alphanumeric string for another field in the same table (commentUID), such that the combined userID+commentUID is unique. Note that commentUID by itself is not necessarily unique - 2 or more users with different userIDs can have the SAME commentUID value.
Background: I have a users table, and a comments table. I need a unique combination for: the user's username (or ID) and the Unique ID of the comment they leave, so that every comment in the system can be uniquely identified in the URL, while also keeping the comment UIDs (and therefore URLs) as short as possible.
The unique ID for the comment should be 4 alphanumeric characters, because each user is not likely to leave more than 36^4 = 1.6M comments (36 denotes all possible values from a-z0-9, lowercase only). An example of this is actually dev.to. See https://dev.to/mindplay/comment/dm5i
How can I do this as a pure MySQL-only solution, such that the commentID is generated when a new record is inserted into the comments table? If you know how to do this, your thoughts are appreciated.