Quote Originally Posted by gmaxey View Post
Why is the GROUP BY line necessary? What does Nz mean and what does it do?
The GROUP BY is necessary because you want to add (group) the extensions of a project together. So group and add per ProjID.

For some projects there are no extensions. So NumDay and the sum of that will return blank values (Null). The Nz (NullToZero) function will convert the nulls to zeroes, so that you can also calculate with it.