Load/transform a simple ID code where field is Null
I am linking to a SQL database. I want to load a simple ID to all loaded lines to represent the source of the data. The content of one of the fields loaded is always Null, so it makes sense to use this field. Is it possible to transform these Null value field lines into ones that contain my source ID? All lines would contain the same ID but I want to identify lines with this source ID later in a visualisation.
Seems simple but I'm not getting anywhere! Can anybody point me in the right direction?