One fun thing you can do in MariaDB is define a collation based off of the Unicode confusables list. The result of this is that select * from usernames where user = 'John' would still work :)
It also avoids registering both 'John' and 'John'. (changed the o)
That's a great feature. I was running a community site once where impersonation became a popular game. So we had dozens of users registering accounts with Cyrillic i,o,a,e etc.
For looking up relevant further material, this is called a 'homograph' or 'homoglyph attack'.
It was also popular for site-spoofing by sending links with these cyrillic character to lead people to a fake bank/search_engine page, until that became widely used, and then owners of the tech-stack (browsers, registrars, dns-operators, etc) shut down this attack vector mostly.
It also avoids registering both 'John' and 'John'. (changed the o)
Hedgewars user registration does this.