--比如你是
select *  from tb where flag >0
--覆盖原有tb的数据
--你可以直接

delete from tb  where flag<=0

select  *  from tb --就是你需要的flag>0的数据
--或者入楼上老大说的

select *  into #temp  from tb where flag>0

delete * from tb

insert into tb select *  from #temp