Search for data cloud db

im using cloud db to save users nickname and if youre first time you have to create nickname and i want to do that if someone try to use nickname as someone else nickname then deny him to do this
(just want to search if nickname exist)
and 1 more thing
i nickname to be max 12 characters long and idk how to do this


Programming Your App to Make Decisions explains how to use if..else statements.

and use is in list

If thing is one of the elements of the list, returns true; otherwise, returns false.

to check if the nickname is already in use.

What have you tried so far?