Något i den här stilen skulle fungera.
with a as
(
SELECt TOP 10 Score
, Post.ID as PostID
FROM Post
order by Score desc
), b as
(
select PostID
, ID as CommentID
, ROW_NUMBER() over (partition by PostID order by ID) as RowNum
from PostComment
)
select *
from a
left join b
on b.PostID = a.PostID
where b.RowNum <= 10