If your mysql seems broken and giving errors like:

# service mysql start
start: Job failed to start

or

ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

Or if you try to recreate databases with mysqlinstalldb:

...
...
ERROR: 1  Can't create/write to file './mysql/db.frm' (Errcode: 13)
041226 xx:xx:xx /usr/libexec/mysqld: Can't find file: './mysql/db.frm' (errno: 13)

or all your databases go missing, even if you reinstall mysql-server.

and reinstalling mysql-server won’t work,

Before you reformat the server and use your data, you should try doing (as root, or using sudo):

apt-get install --reinstall mysql-server

chown -R mysql:mysql /var/lib/mysql

I don’t know how, but somehow the permissions get changed and mysql can no longer operate, or a reinstalled mysql still can’t see the tables. They are not gone, they are there. mysql can’t see them and that can be fixed.

Note: Some complained that if they do:

apt-get install mysql or apt-get remove mysql

They get the error that the package can’t be found. Something like:

# apt-get remove mysql
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package mysql

That has nothing to do with this. This happens because the package for mysql in the repositories is mysql-server and not mysql alone. Doing apt-get install mysql-server will work.