Generate a random value that doesn't exist in the same column

I already googled it, but I just found similar answers.
I'm not a back-end developer, so I don't know a lot of SQL. I need to generate and insert a 6 digit random value that involves numbers and characters (varchar, I think) that doesn't exist in the same column.

I tried this FLOOR(RAND() * 401) + 100 that I found somewhere, but it just generate numbers and... nothing more.

I'm lost.

Please, masters of databases, light my way with your random numbers ;-;