I believe the sessions table contains only current sessions. So if no one is logged in then no session id will appear.
Thanks for your response John.
Yes, I've got access and you are correct, the sessions table only holds the data for current sessions. To work round this I am taking a distinct count of the "historical_events"."id" where "historical_event_types"."type_id" = 1 (server login).
I would be interested if anyone has measured this similarly or used a different method. I am surprised that this is not available in the hist_ tables as they are so comprehensive with other metrics.
I've gone for a similar approach as well. Created a SessionID field that is calculated by most recent log-in prior to a view and converting the datetime to a string and adding the User ID to ensure it's distinct. Surprised they don't keep historical session data.