MySQL 版本多, 各版設定又有差別.
官方文件 http://dev.mysql.com/doc/refman/5.6/en/resetting-permissions.html
If you assigned a root password previously but have forgotten it, you can assign a new password. The following sections provide instructions for Windows and Unix and Unix-like systems, as well as generic instructions that apply to any system.
B.5.3.2.1 Resetting the Root Password: Windows Systems