Your h-index is the highest number n, such that you have at least n questions/answers, all of which have at least n score.
Q&A about the site for people organizing, marketing or licensing open source development projects
select max(p.score) from posts as p
cross apply (select 1 as xyzzy from posts as p2 where (p.owneruserid=p2.owneruserid and p.score<p2.score)) p3
--inner join posts as p2 on (p.owneruserid=p2.owneruserid and p.score<p2.score)
group by p.score, p.id
order by p.score desc