yes, you're right.
but now I really don't know what's should I do
I run this
Code:
apt install mysql-server
I get this
Code:
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-server : Depends: mysql-server-5.5 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
then I run this
Code:
apt install mysql-server-5.5
I get this
Code:
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-server-5.5 : PreDepends: mysql-common (>= 5.5.43-0+deb8u1) but it is not going to be installed
Depends: mysql-client-5.5 (>= 5.5.43-0+deb8u1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
then I run this
Code:
apt install mysql-common
I get this
Code:
Reading package lists... Done
Building dependency tree
Reading state information... Done
mysql-common is already the newest version (5.8+1.0.3).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
then I run this
Code:
apt install mysql-client-5.5
I get this
Code:
Reading package lists... Done
Building dependency tree
Reading state information... Done
mysql-client-5.5 is already the newest version (5.5.43-0+deb8u1).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
I've been really confused 
What's wrong?
thanks