select count(*) as [TotalUsers]
, sum(case when firstPostId > 0 then 1 else 0 end) as [UsersThatPosted]
, sum(case when PostCount > 1 then 1 else 0 end) as [UsersThatPostedMoreThanOnce]
from
(
select id as UserId,
(select top 1 id from posts where owneruserId = users.Id order by creationdate) as [FirstPostId],
(select count(*) from posts where owneruserId = users.Id) as [PostCount]
from users
) as userstats