Question

我有两张桌子：

Table A

id      name
---     -----
1       Foo
2       Bar
3       Fred
4       Joe


Table B
id      tablea_id   result  date
---     ----        ----    ---
1       1           A       02/04/2015
2       2           A       02/04/2015
3       1           B       03/04/2015
4       1           C       04/04/2015

我如何能够获得看起来像这样的结果集？为了“循环”表A中的内容，我需要加入什么？

date        tablea_id   result
---         ---         ----
02/04/2015  1           A
02/04/2015  2           A
02/04/2015  3           NULL
02/04/2015  4           NULL
03/04/2015  1           B
03/04/2015  2           NULL
03/04/2015  3           NULL
03/04/2015  4           NULL
04/04/2015  1           C
04/04/2015  2           NULL
04/04/2015  3           NULL
04/04/2015  4           NULL

Answer 1

您希望cross join生成所有行，并希望left join引入现有值：

select d.date, a.tablea_id, b.result
from a cross join
     (select distinct date from b) d left join
     b
     on b.tablea_id = a.tablea_id and b.date = d.date
order by d.date, a.tablea_id;

Answer 2

我需要什么加入

OUTER JOIN并且最有可能（根据您发布的预期结果）LEFT OUTER JOIN

Answer 3

select ab.id,ab.date,b.result
from
(select a.id,b.date
from(select distinct id
from table_a)a,
(select distinct date
from table_b)b)ab
left outer join 
table_b b
on ab.id=b.id
and ab.date=b.date

Answer 4

使用Full Outer Join检查此简单查询。 Live Demo

            select distinct a.date,a.id as tablea_id ,result from B b
            full outer join
            (
            select a.id,b.date from B b,A a
            )A on  a.id=b.tablea_id and a.date=b.date

我应该用什么样的mysql连接来实现这个目的？

4 个答案: