select a.flast,b.*
from (
select top 20 blockid,本话题树的最后回复贴的时间 as flast
from 文章表
where 第零层=true and BlockID>xxx
) a
left join 文章表 b
on a.blockid=b.blockid
order by a.flast desc --先按话题树排序
, b.... --同话题树的按树结构展开排序