Usar admin en vez de root
First you have to know that Plesk Panel rename the root account when installed by admin.
to have the root mysql unpriviledged logueamos us only as admin. The password is the same as the plesk admin panel
But if you do not have that password …
recover the admin password
versions 8.x, 9.xy bottom that password is in the / etc / psa / .psa.shadow
uses this command to recuperarla
cat /etc/psa/.psa.shadow
from version 10 password is encrypted, command that can exit the text readable
/usr/local/psa/bin/admin --show-password
then the torque command line login into mysql is classical
mysql -uadmin -p`cat /etc/psa/.psa.shadow`
SOURCE: http://munck.fr/es/noticias/como-encuentrar-el-password-mysql-root-en-plesk