https://dev.mysql.com/doc/refman/5.7/en/system-schema.html
MySQL 有一個系統database 就叫mysql, 裡面有個table,
user: User accounts, global privileges, and other nonprivilege columns.
只要匯出這個table, 自行匯入到另一個.
# backup
mysqldump -u root -p --all-databases > backup.sql;
# restore
mysql -u root -p < backup.sql;