帮我解决这个 MySql 完全外部联接（或联合）

Question

这来自将 MSSQL 转换为 MySql。以下是我试图开始工作的代码：

CREATE TEMPORARY TABLE PageIndex (
  IndexId int AUTO_INCREMENT NOT NULL PRIMARY KEY,
  ItemId VARCHAR(64)
);

INSERT INTO PageIndex (ItemId)
SELECT Paths.PathId
  FROM Paths,
       ((SELECT Paths.PathId
           FROM AllUsers, Paths
          WHERE Paths.ApplicationId = @ApplicationId
            AND AllUsers.PathId = Paths.PathId
            AND (@Path IS NULL
                OR Paths.LoweredPath LIKE LOWER(@Path))) AS SharedDataPerPath
          UNION -- This used to be a FULL OUTER JOIN but MySQL doesnt support that.
        (SELECT DISTINCT Paths.PathId
           FROM PerUser, Paths
          WHERE Paths.ApplicationId = @ApplicationId
            AND PerUser.PathId = Paths.PathId
            AND (@Path IS NULL
                OR Paths.LoweredPath LIKE LOWER(@Path))) AS UserDataPerPath
             ON SharedDataPerPath.PathId = UserDataPerPath.PathId)
          WHERE Paths.PathId = SharedDataPerPath.PathId OR Paths.PathId = UserDataPerPath.PathId
          ORDER BY Paths.Path ASC;

假设任何变量已经存在。这在“As SharedDataPerPath”部分出现问题，所以我猜测我给 select 语句加上别名，以便您可以像 MySQL 不支持表一样访问它？如果表架构有帮助，请回复评论，我会将其添加到问题中。

提前致谢！

Answer 1

上述将左右外连接联合在一起的方法效果很好，似乎是普遍接受的解决方案。但是，我在阅读留言板上的各种示例时发现了一些细节。

1. 将用于连接的列上的表源从一个选择交换到另一个选择，以解决外部连接产生的 NULL。

2. 将 COALESCE 函数添加到您的“固定列”中，这些函数也可能以外部连接产生的 NULL 形式返回。

例子：

SELECT
`Wins_VW`.`Year`,
`Wins_VW`.`Period`,
COALESCE(`Wins_VW`.`Wins`,0) as Wins,
COALESCE(`Leads_VW`.`Leads`,0) as Leads
FROM `Wins_VW` LEFT OUTER JOIN `Leads_VW`
ON( `Wins_VW`.`Year` = `Leads_VW`.`Year`
AND `Wins_VW`.`Period` = `Leads_VW`.`Period`)

UNION

SELECT
`Leads_VW`.`Year`,
`Leads_VW`.`Period`,
COALESCE(`Wins_VW`.`Wins`,0) as Wins,
COALESCE(`Leads_VW`.`Leads`,0) as Leads
FROM `Wins_VW` RIGHT OUTER JOIN `Leads_VW`
ON( `Wins_VW`.`Year` = `Leads_VW`.`Year`
AND `Wins_VW`.`Period` = `Leads_VW`.`Period`)

Answer 2

-- Assuming these are defined in your store procedure
DECLARE @ApplicationId VARCHAR(64);
DECLARE @Path VARCHAR(256);
SET @ApplicationId = NULL;
Set @Path = NULL;

CREATE TEMPORARY TABLE SharedDataPerPath
(
  PathId VARCHAR(256)
);

CREATE TABLE UserDataPerPath
(
  PathId VARCHAR(256)
);

-- Do this instead of aliasing a select statment 'AS SharedDataPerPath'
INSERT INTO SharedDataPerPath
SELECT Paths.PathId
  FROM aspnet_PersonalizationAllUsers AllUsers, aspnet_Paths Paths
 WHERE Paths.ApplicationId = @ApplicationId
   AND AllUsers.PathId = Paths.PathId
   AND (@Path IS NULL OR Paths.LoweredPath LIKE LOWER(@Path));

-- Do this instead of aliasing a select statement 'AS UserDataPerPath'
INSERT INTO UserDataPerPath
SELECT DISTINCT Paths.PathId
  FROM aspnet_PersonalizationPerUser PerUser, aspnet_Paths Paths
 WHERE Paths.ApplicationId = @ApplicationId
   AND PerUser.PathId = Paths.PathId
   AND (@Path IS NULL OR Paths.LoweredPath LIKE LOWER(@Path));

-- This is how I would do my 'FULL OUTER JOIN'
SELECT Paths.PathId
    FROM `wppi_net_db`.`aspnet_Paths` Paths,
         (SELECT *
            FROM SharedDataPerPath AS s
            LEFT OUTER JOIN UserDataPerPath AS u
              ON s.PathID = u.PathID
           UNION -- OR UNION ALL see: http://www.xaprb.com/blog/2006/05/26/how-to-write-full-outer-join-in-mysql/
          SELECT *
            FROM SharedDataPerPath AS s
           RIGHT OUTER JOIN UserDataPerPath AS u
              ON s.PathID = u.PathID) AS DataPerPaths
   WHERE Paths.PathId = DataPerPaths.PathId
   ORDER BY Paths.Path ASC;

-- At some point you need to drop your temp tables
DROP TEMPORARY TABLE SharedDataPerPath;
DROP TEMPORARY TABLE UserDataPerPath;

Answer 3

FULL OUTER JOIN 通常可以用LEFT JOIN 和RIGHT JOIN 的UNION 模拟。即它都在左边和右边，尽可能匹配连接条件。根据我的经验，它通常很少使用。我有一个只使用一次的大型系统。

由于FULL OUTER JOIN 不可用，您似乎想要在这里做的是UNION 两组并在两个子集之间设置一些JOIN 标准，这确实是不可能的。在您的示例中，UNIONed 的两个集合不能有别名，也不能有尝试链接它们的 WHERE 子句。