查询具有 2 个外键的表

Question

我有这 4 张桌子。

Mail
----
id
date

Source (can have many source)
------
id
mailID : FK Mail.id
personID : FK Person.id

Destination  (can have many destination)
-----------
id 
mailID : FK Mail.id
personID : FK Person.id

Person
------
id
name 

我正在尝试创建一个查询，该查询返回包含所有源和目标 ID 的每封邮件。

到目前为止，我已经提出了这个查询，但它的效率相当低，因为它查询同一个表两次。

Select * FROM (
    Select m.*, personID, 'Source' AS TableName
    FROM Mail m join Source s 
    ON m.id = s.mailid
UNION ALL
    Select m.*, personID, 'Destination' AS TableName 
    FROM Mail m join Destination d
    ON m.id = d.mailid


) t ORDER BY id, TableName;

results:

id | date | personID | TableName
--------------------------------
 1 | 1-1-11| 3       | Source
 1 | 1-1-11| 4       | Source
 1 | 1-1-11| 5       | Source
 1 | 1-1-11| 10      | Destination
 1 | 1-1-11| 11      | Destination
 2 | 2-2-11| 1       | Source
 2 | 2-2-11| 2       | Destination

Answer 1

试试这个：

SELECT m.id, m.name, a.personID, a.TableName
FROM Mail m
INNER JOIN (SELECT s.mailid, s.personID, 'Source' AS TableName
            FROM Source s 
           UNION ALL
            SELECT d.mailid, d.personID, 'Destination' AS TableName
            FROM Destination d
           ) AS a ON m.id = a.mailid
ORDER BY m.id, a.TableName;