My attempt to install MySql over the remains of earlier versions of MySql and MariaDB is for purposes of illustration only. It is the only way to get the thing to admit the existence of broken packages. Thus:-

loz@loz-Comp:~$ sudo apt-get install MySql-Client
[sudo] password for loz:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies.
mysql-client : Depends: mysql-community-client (= 5.6.20-1ubuntu14.04) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

The beginning of this problem is me trying to install WordPress. It demanded installation of one of a very limited list of SQL's, and I picked Maria, unaware that MySQL was already there. I download the DEB package, double-click on it, follow installation instructions, and the broken package appears. Also notification of system problems. I do not know if it is a good idea to attempt to re- install Maria DB. Thanks.

Currently you have on the machine a mix of MySQL 5.5.38, MySQL 5.6.19, MariaDB 10.1.0, remains of i386 installation which apparently doesn't belong there at all, and you also manually removed a package from the cache. On top of it, you are trying to install MySQL client 5.6.20. I don't think it's surprising that apt-get complains about broken packages; I also doubt that the whole mess happened upon installation of MariaDB 10.1.0 only, it wouldn't pull different versions of MySQL.

I assume you already tried to run apt-get install -f ? Try it several times, stupid as it sounds, it's known to work this way.
Also, if you just want to get rid of MariaDB packages, try to uninstall (apt-get remove) mariadb-common 10.1.0+maria-1~trusty and mysql-common 10.1.0+maria-1~trusty. It will most likely cause removal of some other packages, but since your installation is currently broken anyway, it's not a big loss. No data should be removed unless you also purge the packages, but a backup is recommended anyway.

Regardless the package problem that you are experiencing, should you want to try MariaDB again in a more controlled environment, please keep in mind that 10.1.0 is currently Alpha and is not recommended for use in production. Please use 10.0 instead.

I am facing same Problem. MariaDB is working fine. but i can not use apt-get command anymore. apt-get autoremove -f , apt-get dist-upgrade nothing works.

$sudo apt-get autoremove -f
The following extra packages will be installed:
libmysqlclient18
The following packages will be upgraded:
libmysqlclient18

dpkg: error processing archive /var/cache/apt/archives/libmysqlclient18_10.1.0+maria-1~trusty_amd64.deb (--unpack):
libmysqlclient18:amd64 10.1.0+maria-1~trusty (Multi-Arch: no) is not co-installable with libmysqlclient18 which has multiple installed instances
Errors were encountered while processing:
/var/cache/apt/archives/libmysqlclient18_10.1.0+maria-1~trusty_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
...........

$ dpkg -l | grep -E 'maria|mysql'

ii libapache2-mod-auth-mysql 4.3.9-13.1ubuntu3 amd64 Apache 2 module ...
ii libdbd-mysql-perl 4.025-1 amd64 Perl5 database i...
iU libmariadbclient18 10.1.0+maria-1~trusty amd64 MariaDB database...
ii libmysqlclient18:amd64 5.5.38-0ubuntu0.14.04.1 amd64 MySQL database c...
rc libmysqlclient18:i386 5.5.38-0ubuntu0.14.04.1 i386 MySQL database c...
ii libmysqlcppconn7 1.1.3-5 amd64 MySQL Connector ...
ii libqt4-sql-mysql:amd64 4:4.8.5+git192-g085f851+dfsg-2ubuntu4 amd64 Qt 4 MySQL datab...
iU mariadb-client-10.1 10.1.0+maria-1~trusty amd64 MariaDB database...
iU mariadb-client-core-10.1 10.1.0+maria-1~trusty amd64 MariaDB database...
ii mariadb-common 10.1.0+maria-1~trusty all MariaDB database...
ii mysql-client 5.5.38-0ubuntu0.14.04.1 all MySQL database c...
ii mysql-common 5.5.38-0ubuntu0.14.04.1 all MySQL database c...
rc mysql-server-5.5 5.5.38-0ubuntu0.14.04.1 amd64 MySQL database s...
ii mysql-utilities 1.3.5-2 all collection of sc...
ii mysql-workbench 6.0.8+dfsg-2 amd64 MySQL Workbench ...
ii mysql-workbench-data 6.0.8+dfsg-2 all MySQL Workbench ...
ii php5-mysql 5.5.16+dfsg-1+deb.sury.org~trusty+1 amd64 MySQL module for...
ii python-mysql.connector 1.1.6-1 all pure Python impl...
ii python-mysqldb 1.2.3-2ubuntu1 amd64 Python interface...

L C Wyche, nafis,

To offer a sensible explanation and exact steps to fix the environment, we need to know how you got there.
At some points, you had at least

• MySQL 5.5 installed from Ubuntu repo;
• MariaDB 5.5 installed from Ubuntu repo;
• MySQL 5.6 installed from the customized MySQL repo;
• MariaDB 10.1 installed from the official MariaDB repo;
  possibly something else.

It also doesn't help that both of you have some i386 packages installed alongside with amd64. I've seen before mysql-common:i386 coming from MySQL repo along with MySQL 5.6 server (amd64), don't know the conditions why it happens. Possibly libmysqlclient18:i386 came from the same source.

I know for a fact that switching between versions and distributions like that causes various problems on deb-based systems, but to know which flavor of the problem you hit, it's necessary to understand in which order you were installing the versions and, very importantly, how you were doing it (exact apt-get commands: whether it was install, or upgrade, or dist-upgrade, and if it was install, which packages you listed explicitly).

Without knowing more, I can only make some general suggestions.

1) As already mentioned (none of you said whether it helped or not), try to manually remove all packages starting from mysql-, mariadb-, libmysql* and libmariadb* (and even purge if possible). Maybe it will help to fix the broken environment. After you removed everything, you can also retry install -f.

2) After it is fixed,

• if you want to have MariaDB 5.5, remove from source lists both MySQL 5.6 repo and MariaDB repo, and install mariadb-server from the Ubuntu repo.
• if you want to have MySQL 5.5, remove from source lists both MySQL 5.6 repo and MariaDB repo, and install mysql-community-server from the Ubuntu repo.
• if you want to have MariaDB 10.0 or 10.1, remove from source lists MySQL 5.6 repo, add MariaDB 10.0 or 10.1 repo and install mariadb-server.
• if you want to have MySQL 5.6, remove from source lists MariaDB repo and install mysql-server.

Whichever you choose, you should never have both MySQL 5.6 repo and MariaDB repo enabled at the same time. It really makes no sense because there is nothing else useful in the repos apart from the servers/clients, and you cannot have both servers installed simultaneously.

Closing as incomplete for now (see the previous comment).
If you have more information, please comment to re-open.