% Active By Month Telematics

[dmac1971]

I'm working though some telematics data and want to be able to work out the total pop of machines counting all machines from the first date they sent data until today.

Not all machines work every day, which is fine, but I want to show % of machines that worked during a month, vs all the machines that "could" have worked.

What is the simplest way to do this.  I'm assuming I load all data, then reload a separate table with firstdate only and then use this to create a count of all possible machines. Is there a way to do this without multiple loads.

Machines are identified by a unique serial number.