select avg(count(ip)) from pagehits where [month] = 2 group by ip
will give the following error: “Cannot perform an aggregate function on an expression containing an aggregate or a subquery.” MS SQL Server doesn't support it.
Solution - use a derived table:
select avg(ipcount) from (select count(ip) ipcount from pagehits where [month] = 2 group by ip) as sub
I'm posting this because searches on the error message didn't return good results, so if someone else has this problem (read: when I forget this again) this may save some frustration.