问求解一个sql查询语句？

select top 50 * from pagetest

where not exists

(select 1 from (select top 9900 id from pagetest order by id)a where a.id=pagetest.id)

order by id

pagetest是表名 a是别名吗？ 要是别名的话怎么分的页啊？可以详细介绍一下嘛？