MySQL中有个表:article(字段:id,type,date),type有1-10,10种类型。现在要用SQL找出每种类型中时间最新的前N个数据组成的集合。
select a1.* from article a1
inner join
(select a.type,a.date from article a left join article b
on a.type=b.type and a.date<=b.date
group by a.type,a.date
having count(b.date)<=2
)b1
on a1.type=b1.type and a1.date=b1.date
order by a1.type,a1.date desc
就想要问下 a.date<=b.date 是什么意思
count(b.date)<=2又是啥意思
orcal
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