1.分页 limit offset 和排序 order by desc 降序 asc 升序
select a.1,2,3,4 from a
inner join b
on a.1=b.1
inner join c
on c.2=b.2
where 3='xxx'
order by 4 desc (asc);
2.分页是所有select where条件的最后一项； limit 起始值，行数
limit （n-1）*pageSize ,pageSize 页数=数据总数/pageSize ---pageSize 页面大小；
select a.1,2,3,4 from a
inner join b
on a.1=b.1
inner join c
on c.2=b.2
where 3='xxx'
order by 4 desc (asc)
limit 0,10；