I am running in the same Linux distro two environments: Plasma (former KDE) and i3wm.

I use the same command on both environments: sudo systemctl start mysql.service && sudo mysql -u root -p -h localhost -P 3306, but while on KDE/Plasma I get no error, on i3wm I get a "the name is not activatable" error, and I don't know how to fix it (here's a screenshot).

Here's an extract from MySQL Workbench:

04:32:53 [ERR][SQL Editor Form]: SQL editor could not be connected: The name is not activatable
04:32:53 [ERR][SQL Editor Form]: Your connection attempt failed for user 'root' to the MySQL server at 127.0.0.1:3306:
  The name is not activatable

Also, I found out firefox doesn't connect to 127.0.0.1:3306 either on i3; on Plasma I get something like ("The connection has been restarted. The connection to the server was reset while the page was loading.").

I find is interesting too, because syncthing can connects through the localhost on i3wm, but mariadbd can't for some reason:

sudo lsof -n | grep TCP | grep LISTEN
[…]
syncthing   841 31843 syncthing          alberto  14u     IPv4              11863       0t0        TCP 127.0.0.1:8384 (LISTEN)
mariadbd   2537                            mysql  32u     IPv4              16382       0t0        TCP *:mysql (LISTEN)