SQL Server vs MySQL vs PostgreSQL – database engine version

Firstly, I would like to know what is the version of the database engine installed on the server/workstation. We all know SQL Server, it’s all provided by @@version variable, but how to get this info in MySQL or PostgreSQL?
Just to remind, SQL Server:

Sample output below, it provides the details about both, the database and the operating system:

MySQL has 2 variable available, one the same as SQL Server:

and variable version():

Both will return the same output, ie:

PostgreSQL:

Sample output:

r2db – different views at the data