学习关于聚合FUNCTION（count(*|fieldname)）

 -----------------结果集为刚好分组后每组中记录数为3个
select * from b t  
where (select count(*) from b where a = t.a and b > t.b) <3
 and exists(select count(*) from b where a=t.a group by a having count(*)>=3)
go
----------------结晶集为分组后每组中记录数为》=3个
select *
from b t
where exists(select count(*) from b where a=t.a group by a having count(*)>=3)