SQL - White Christmas - solution

4.
A person's White Christmas Count (wcc) is the number of White Christmases they were exposed to as a child 
(between 3 and 12 inclusive assuming they were born at the beginning of the year and were about 1 year old on their first Christmas).
Charles Dickens's wcc was 8.
List all the years and the wcc for children born in each year of the data set. Only show years where the wcc was at least 7.

Paste and run at:
http://sqlzoo.net/wiki/White_Christmas

select a.yr as year, sum(b.white) as wcc
from
(select distinct yr from hadcet) a
join
(select yr, case when min(m12/10)<0 then 1 else null end as white 
from hadcet
where dy between 21 and 25
group by yr) b
on b.yr between a.yr + 2 and a.yr + 11
group by a.yr
having sum(b.white) >=7