Install

Ubuntu 18.04 (Bionic)

vi /etc/apt/sources.list.d/pgdg.list

add...

deb http://apt.postgresql.org/pub/repos/apt/ bionic-pgdg main

Import the repository signing key, and update the package lists 

wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -

sudo apt update

Install...

apt install postgresql-client-11

apt install postgresql-client-12