MySQL菜鸟问题联合查询

Question

a 表
    userid  name  areaid
       1         1        1
       2         2        2
       3         3        3
b表 发表的文章
      contentid   content  userid
          1             asdf       1
          2            sdfsd       2
          3            dsfsdsd    3
          4           ^^^^^^    1
          5           ^^^^^^^  2
          6           @@@@@    3
          7           #######   1
          8            #######  2
          9           #######   3
          10           #######  3现在我想要 三个用户发表的三篇文章 
              uerid    contentid
                1           1
                 1          4
                 1          7
                2           2
                2           5
                2           8
                3           3
                3           6
                3           9
这个怎么才能成为这样的结果呢 

Answer 1

SELECT
    userid,contentid
FROM b表
ORDER BY userid;

Answer 2

select userid,contentid from b 
left join a on a.userid=b.userid
order by userid