我这样删除了当前的mysql：
apt-get remove --purge mysql-server mysql-client mysql-common
apt-get remove --purge mysql-client-core-5.5
apt-get autoremove
apt-get autoclean
rm -rf /var/lib/mysql
当我想重新安装时，我遇到了一些问题：
apt-get install mysql-server
//输出是
The following packages have unmet dependencies:
mysql-server : Depends: mysql-server-5.5 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.**
然后再次
apt-get install mysql-server-5.5
//输出是
以下信息可能有助于解决问题：
The following packages have unmet dependencies:
mysql-server-5.5 : PreDepends: mysql-common (>= 5.5.47-0+deb7u1) but it is not going to be installed
Depends: mysql-client-5.5 (>= …