The main username and password are the same for cPanel login, FTP login, and MySQL database access.
When you change your cPanel password, this also changes your main FTP password and main database password. Any FTP users and Database users you created are not affected. Email accounts you created are not affected.
NEVER, EVER, use your Cpanel username & password for database connections -- it is highly insecure & makes your account susceptible to nasty hackery. Don't eat where you poop, make sure you have unique accounts set up for each user, application, and database at varying levels of permissions & please use unique passwords for each.