How many users have some tweets?
select count(*) from user_profiles where uid in (select uid from tweets)
How about the number of users that have tweeted more than once?
select count(*) from (select count(*) from user_profiles as t1 join tweets as t2 on t1.uid = t2.uid group by t2.uid having count(*) > 1)
Show their names and number of tweets tweeted by them.
select t1.name, count(*) from user_profiles as t1 join tweets as t2 on t1.uid = t2.uid group by t2.uid having count(*) > 1