Install MySQLMan
Use that to edit the 'mysql' table directly.
Then, reload mysql to let the changes take effect. This is the _biggest_ problem. People forget that if you edit the permissions directly you MUST issue a 'reload' command. If you use 'grant' the reload is done automatically, but the syntax is much more complex.
It's really easy to add a user, and set _all_ their permissions to 'n' (except the root user) in the user table. This way, a user exists, but has _no_ access to anything.
Then, go to the database table, and enter that user, and give them permissions to access that database you want to give them access to, and give them the necessary permissions for that database in that table.
That way, your users have acces _only_ to the specific database, and can't do anything else. DO NOT give them "grant" ability (but I think you can do that only in the 'users' table anyway.
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