列名不明确的 MySQL 完全连接（联合）

Question

我对 MySQL 在这么多年后仍然不支持 FULL JOIN 感到非常失望（为什么？）并且我必须使用 UNION 作为解决方法。说我在以下查询中遇到问题。我有两个表作为例子：

计算机

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

监视器

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

基本上，我有一个已分配给所有者的具有规格的计算机列表。同样，我有一个带有规格的监视器列表，每个监视器都分配给一个所有者。我想按所有者将这两个表组合成这个结果：

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

我可以通过这个查询成功得到上面的结果：

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：

1. 我不想在我的查询中使用 jolly 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
2. 两个表都有不明确的列名（id、userid、owner），例如，我似乎不能按所有者排序。我知道我应该使用别名，但我不明白如何实现它
3. 我必须使用 WHERE 语句，但由于列名不明确，我不明白如何实现它

现在总结一下我在伪查询中所说的话：

选择 id 作为 a_id，用户 ID 作为 a_userid，所有者作为 a_owner 但不是 cpu 来自 cpu = 3.4 的计算机

由所有者联合和加入：

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

感谢您的宝贵时间。

Answer 1

这是 cpu = 3.4 和 Inch = 22 上的完全外连接。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。主表条件在 WHERE 子句中，外连接条件在 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未找到的记录来加快速度，即没有匹配计算机的监视器。然后您可以使用 UNION ALL 代替 UNION，这可以为 dbms 节省大量工作。您只需拿起外部连接表的不可为空的线圈。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;

Answer 2

模拟full outer join 的另一种方法是使用子查询获取完整的值列表（在您的情况下为owners）。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。它将为“Jack”生成四个结果。

编辑：

我怀疑你想匹配 owner、id 和 user_id：

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;