select a.classid,a.count,b.* from A表 a inner join(
select *,row_number()over(partition by classid order by newid())rn from B表
)b on a.classid=b.classid
where b.rn<(select count(*)*0.7 from B表 where classid=b.classid)

------解决方案--------------------
SQL code
select distinct n.* from B m
cross apply(select top percent 70 * from B where classid=m.classid) n

------解决方案--------------------
探讨


SQL code

select a.classid,a.count,b.* from A表 a inner join(
select *,row_number()over(partition by classid order by newid())rn from B表
)b on a.classid=b.classid
where b.rn<(select count(*)*0.7 from……


------解决方案--------------------
各位请注意一点：select top percent 70 * from B where classId=‘A类’

这个百分比写的有点问题
修正：
 select top 70 percent  * from B where classId=‘A类’