Question

我很失望的事实是MySQL在这么多年后仍然不支持FULL JOIN（为什么呢？）而且我必须使用UNION作为解决方法。说我遇到以下问题的问题。我有两张桌子作为例子：

计算机

id   userid   owner   cpu   
1    1        Jack    3.4
2    1        Jack    3.4
3    2        Sara    3.0

显示器

id   userid   owner   inch  
1    1        Jack    22    
2    1        Jack    22    
3    3        Mark    17    
4    4        Luke    32

基本上我有一个计算机列表，其中包含已分配给所有者的规范。同样，我有一个带有规格的监视器列表，每个监视器分配给一个所有者。我希望按所有者将这两个表合并到这个结果中：

id   userid   owner   cpu   id   userid   owner   inch  
1    1        Jack    3.4   1    1        Jack    22
2    1        Jack    3.4   2    1        Jack    22
3    2        Sara    3.0   null null     null    null 
null null     null    null  3    3        Mark    17
null null     null    null  4    4        Luke    32

我可以使用此查询成功获得上述结果：

SELECT * FROM computers AS a
LEFT OUTER JOIN monitors AS o on a.owner = o.owner
UNION
SELECT * FROM computers AS a
RIGHT OUTER JOIN monitors AS o on a.owner = o.owner

问题是：

我不希望在我的查询中使用快乐字符，不仅因为它效率不高，而且因为我只需要一些特定的列而不是全部
两个表都有不明确的列名（id，userid，owner），所以例如我似乎不能通过所有者进行ORDER BY。我知道我应该使用别名，但我不明白如何实现它
我必须使用WHERE语句，但由于列名不明确，我不知道如何实现它

现在总结我在伪查询中所说的内容：

SELECT id AS a_id，userid AS a_userid，owner AS a_owner 但不是 cpu FROM计算机WHERE cpu = 3.4

所有者加入UNION和JOIN：

SELECT id AS b_id，userid AS b_userid，owner AS b_owner，inch AS b_inch FROM monitor WHERE inch = 22

感谢您的时间。

Answer 1

模拟full outer join的另一种方法是使用子查询获取值的完整列表（owner）。然后使用left outer join引入其他值。根据您的描述：

select c.id as c_id, c.userid as c_userid, o.owner as owner,
       m.id as m_id, m.user_id as m_userid, m.inch
from (select owner from computers union
      select owner from monitors
     ) o left outer join
     computers c
     on o.owner = c.owner left outer join
     monitors m
     on o.owner = m.owner;

但是，您提到的查询不会生成这些结果。它会为“杰克”生成四个结果。

编辑：

我怀疑您希望在owner，id和user_id上匹配：

select c.id as c_id, c.userid as c_userid, o.owner as owner,
       m.id as m_id, m.user_id as m_userid, m.inch
from (select owner, id, userid from computers union
      select owner, id, userid from monitors
     ) o left outer join
     computers c
     on o.owner = c.owner and o.userid = c.userid and o.id = c.id left outer join
     monitors m
     on o.owner = m.owner and o.userid = m.userid and o.id = m.id;

Answer 2

这是cpu = 3.4和Inch = 22的完全外连接。首先获取所有计算机并加入监视器，然后获取所有监视器并加入计算机。主表标准在WHERE子句中，外连接标准在ON子句中。

SELECT a.userid, a.owner, a.id as computer_id, o.id as monitor_id, o.inch
FROM computers AS a
LEFT OUTER JOIN monitors AS o 
  ON a.owner = o.owner and a.userid = o.userid and o.inch = 22
WHERE a.cpu = 3.4
UNION
SELECT o.userid, o.owner, a.id as computer_id, o.id as monitor_id, o.inch
FROM monitors AS o
LEFT OUTER JOIN computers AS a
  ON a.owner = o.owner and a.userid = o.userid and a.cpu = 3.4
WHERE o.inch = 22;

但是，这可以通过仅添加第一个选择找不到的记录来加速，即没有匹配计算机的监视器。然后你可以使用UNION ALL代替UNION，它可以为dbms节省大量的工作。你只需要选择一个外部连接表的不可空的线圈。当它为NULL时，记录是外连接的。

SELECT a.userid, a.owner, a.id as computer_id, o.id as monitor_id, o.inch
FROM computers AS a
LEFT OUTER JOIN monitors AS o 
  ON a.owner = o.owner and a.userid = o.userid and o.inch = 22
WHERE a.cpu = 3.4
UNION ALL
SELECT o.userid, o.owner, a.id as computer_id, o.id as monitor_id, o.inch
FROM monitors AS o
LEFT OUTER JOIN computers AS a
  ON a.owner = o.owner and a.userid = o.userid and a.cpu = 3.4
WHERE o.inch = 22 and o.owner is null;

带有模糊列名的MySQL完全连接（联合）

2 个答案: