数据量大的话，left join相对比较慢，如果是分页显示或者只是求前面几十条的数据，可以先求点赞表排序好了的aid，再在文章表中找这些aid对应的文章

select a.content from article a left join praise b on a.aid=b.aid order by b.time desc

SELECT
  a.aid,
  a.content,
  pr.praiseCount
FROM article a
  LEFT JOIN (SELECT
               p.aid,
               count(1) AS praiseCount
             FROM praise p
             GROUP BY p.aid) pr
    ON a.aid = pr.aid
ORDER BY pr.praiseCount DESC

select a.aid,count(p.aid) num from article a left join praise p on a.aid=p.aid group by p.aid order by num desc;