sql - SQLZOO Nobel Tutorial #8

Question

http://sqlzoo.net/wiki/SELECT_from_Nobel_Tutorial

Can someone give me some help to do the #8?
I am new to SQL, and I really have no idea why my answer is wrong:

select yr from nobel where 
yr in (select distinct yr from nobel where subject='Physics') 
and 
yr not in (select distinct yr from nobel where subject='Chemistry')

Thanks

Answer 1

All, I got it solved by myself.

select distinct yr from nobel 
where 
subject='Physics' and yr not in 
(select distinct yr from nobel where subject='Chemistry')

Thanks

I also find a way to reveal the answer of those questions(at least some of them) just append ?answer=1 to the url, you may be able to get an answer page. Hope this works!