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select top 1 rr.id
from rr,ai
where rr.createtime between '2015-2-15' and '2015-2-16' and rr.userid=ai.userid and ai.a='nice'
order by rr.createtime desc
select top 1 rr.id
from rr,ai
where rr.userid=ai.userid and rr.createtime between '2015-2-15' and '2015-2-16'
order by rr.createtime desc
这样写吗?该写法无论TOP 1是否有值,ai和rr的逻辑读次数和exists写法是一样的