Retrieving Custom Properties via SWQL API

How can I retrieve a Nodes Custom Property via SWQL API?

I have created a Custom Property "Serial_Number" for Nodes in NCM 7.1. I would like to retrieve this via SWQL API, but the following query does not work:

SELECT Serial_Number From Orion.Nodes

It gives an error response: Cannot resolve property Serial_Number

In MSSQL I can see that there is a column Serial_Number in the dbo.Nodes table.