We have at the moment c. 940,000 questions at MSE. Some users have access to statistics of the site.
I'm interested in the issue:
How many questions were asked by one-question users? (users who asked only one question)
SEDE returned 115626 users with exactly one question. The query was
select count(a.uid) from ( select OwnerUserId as uid from Posts where PostTypeId = 1 group by OwnerUserId having count(Id) = 1 ) a
I imagine a large number of such questions are deleted, but those cannot be counted. Even if one uses the table PostsWithDeleted instead of table Posts, this does not help because the rows for deleted posts do not have OwnerUserId filled.