When I try to install mysql-server, an error comes like:
dpkg: error processing package mysql-server (--configure):
dependency problems - leaving unconfigured
Errors were encountered while processing:
mysql-server-5.7
mysql-server
E: Sub-process /usr/bin/dpkg returned an error code (1)
What does it mean. Any ideas?
dpkg returning an error code 1 doesn't mean anything specific but it usually has to do with dependency issues. In your case, you tried to install two versions/instances of the same package i.e. mysql-server and mysql-server-5.7. So do the following to remove any redundant dependency issues and install a functioning mysql package
sudo apt-get clean
sudo apt-get purge mysql*
sudo apt-get update
sudo apt-get install -f
sudo apt-get install mysql-server
sudo apt-get dist-upgrade
this should fix the problem at hand. But in the future, have care about the package names you add after sudo apt-get install since the wrong list of package names - for example redundant entries in the list - results in failure to install either of the packages or worse - you might even find yourself wading through the hellish depths of #DEPENDENCY-HELL