There are several distinctions between the way user names and passwords are
used by MySQL, and the way they are used by Unix or Windows:
User names, as used by MySQL for authentication purposes, have
nothing to do with Unix user names (login names) or Windows user names. Most
MySQL clients by default try to log in using the current Unix user
name as the MySQL user name, but that is for convenience only.
Client programs allow a different name to be specified with the
--user options. This means that you can't make a database secure in
any way unless all MySQL user names have passwords. Anyone may
attempt to connect to the server using any name, and they will succeed if
they specify any name that doesn't have a password.
MySQL user names can be up to 16 characters long; Unix user names
typically are limited to 8 characters.
MySQL passwords have nothing to do with Unix passwords. There is no
necessary connection between the password you use to log in to a Unix machine
and the password you use to access a database on that machine.
MySQL encrypts passwords using a different algorithm than the one
used during the Unix login process. See the descriptions of the
ENCRYPT() functions in 7.3.12 Miscellaneous functions.