Mysql on Ubuntu 16.04

Question

I'm installing Mysql by

sudo apt-get update
sudo apt-get install mysql-server

It is not asking for creating root password during the installation.

When I try to connect to mysql it ask for root's password.

:/

pauloh@mccartney:~$ sudo mysql -u root
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

When I enter blank password:

pauloh@mccartney:~$ sudo mysql -u root -p
Enter password: 
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

When I enter 'root' as password:

pauloh@mccartney:~$ sudo mysql -u root -p
Enter password: 
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)
pauloh@mccartney:~$ 

I once installed and was never asked to enter a root password. when I try to install again no password configuration screen is shown and this is the output:

pauloh@mccartney:~$ sudo apt-get install mysql-server
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libaec0 libgeos-3.5.0 libgeos-c1v5 libhdf4-0-alt libhdf5-10 libnetcdf11
  libpcrecpp0v5 libpq5 libsz2 libxerces-c3.1 libzip4 linux-headers-4.4.0-59
  linux-headers-4.4.0-59-generic linux-image-4.4.0-59-generic
  linux-image-extra-4.4.0-59-generic python-crypto python-ecdsa
  python-paramiko python-six ubuntu-core-launcher
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
  mysql-client-5.7 mysql-client-core-5.7 mysql-common mysql-server-5.7
  mysql-server-core-5.7
Suggested packages:
  mailx tinyca
The following NEW packages will be installed:
  mysql-client-5.7 mysql-client-core-5.7 mysql-common mysql-server
  mysql-server-5.7 mysql-server-core-5.7
0 upgraded, 6 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/18,7 MB of archives.
After this operation, 160 MB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Preconfiguring packages ...
Selecting previously unselected package mysql-common.
(Reading database ... 268323 files and directories currently installed.)
Preparing to unpack .../mysql-common_5.7.17-0ubuntu0.16.04.1_all.deb ...
Unpacking mysql-common (5.7.17-0ubuntu0.16.04.1) ...
Selecting previously unselected package mysql-client-core-5.7.
Preparing to unpack .../mysql-client-core-5.7_5.7.17-0ubuntu0.16.04.1_amd64.deb ...
Unpacking mysql-client-core-5.7 (5.7.17-0ubuntu0.16.04.1) ...
Selecting previously unselected package mysql-client-5.7.
Preparing to unpack .../mysql-client-5.7_5.7.17-0ubuntu0.16.04.1_amd64.deb ...
Unpacking mysql-client-5.7 (5.7.17-0ubuntu0.16.04.1) ...
Selecting previously unselected package mysql-server-core-5.7.
Preparing to unpack .../mysql-server-core-5.7_5.7.17-0ubuntu0.16.04.1_amd64.deb ...
Unpacking mysql-server-core-5.7 (5.7.17-0ubuntu0.16.04.1) ...
Processing triggers for man-db (2.7.5-1) ...
Setting up mysql-common (5.7.17-0ubuntu0.16.04.1) ...
update-alternatives: using /etc/mysql/my.cnf.fallback to provide /etc/mysql/my.cnf (my.cnf) in auto mode
Selecting previously unselected package mysql-server-5.7.
(Reading database ... 268480 files and directories currently installed.)
Preparing to unpack .../mysql-server-5.7_5.7.17-0ubuntu0.16.04.1_amd64.deb ...
Unpacking mysql-server-5.7 (5.7.17-0ubuntu0.16.04.1) ...
Selecting previously unselected package mysql-server.
Preparing to unpack .../mysql-server_5.7.17-0ubuntu0.16.04.1_all.deb ...
Unpacking mysql-server (5.7.17-0ubuntu0.16.04.1) ...
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for systemd (229-4ubuntu16) ...
Processing triggers for ureadahead (0.100.0-19) ...
Setting up mysql-client-core-5.7 (5.7.17-0ubuntu0.16.04.1) ...
Setting up mysql-client-5.7 (5.7.17-0ubuntu0.16.04.1) ...
Setting up mysql-server-core-5.7 (5.7.17-0ubuntu0.16.04.1) ...
Setting up mysql-server-5.7 (5.7.17-0ubuntu0.16.04.1) ...
update-alternatives: using /etc/mysql/mysql.cnf to provide /etc/mysql/my.cnf (my.cnf) in auto mode
Renaming removed key_buffer and myisam-recover options (if present)
Checking if update is needed.
This installation of MySQL is already upgraded to 5.7.17, use --force if you still need to run mysql_upgrade
Setting up mysql-server (5.7.17-0ubuntu0.16.04.1) ...
Processing triggers for systemd (229-4ubuntu16) ...
Processing triggers for ureadahead (0.100.0-19) ...