Question

我想提取有关存储在数据库中的人员和公司的信息。我有2个表（客户和联系人），它们指向包含2个外键（addressClientID和addressContactID）的唯一地址表。客户可以有多个地址，“联系人”通常是一个，它可以是自定义地址（addressContactID将指向联系人ID，否则为NULL）或正在工作的客户的地址。查询应提取联系人的姓名，该联系人正在工作的公司的名称以及城市。后者必须是公司所在的城市（如果addressContactID为NULL）或联系人所在的城市（如果addressContactID指向联系人ID）。

我尝试了多种类型的联接，但均未成功。 DBMS是MySql 5.2

mysql> select * from Clients;
+----+---------+-------------+--------+
| id | name    | taxCode     | optOut |
+----+---------+-------------+--------+
|  1 | Client1 | 51824753556 |      0 |
|  2 | Client2 | 51824543536 |      0 |
+----+---------+-------------+--------+
2 rows in set (0.01 sec)

mysql> select * from Contacts;
+----+--------------+---------------+------------------------+------------------+------------------+--------------+-------+------------------+
| id | name         | preferredName | email                  | mobile           | phone            | jobTitle     | notes | contactsClientId |
+----+--------------+---------------+------------------------+------------------+------------------+--------------+-------+------------------+
|  1 | Contact 1    | NULL          | contact1@gmail.com     | +61 421 111111   | +61 421 22222222 | Title1       | NULL  |                1 |
|  2 | Contact 2    | NULL          | contact2@gmail.com     | +61 421 15345431 | +61 421 263462   | Title2       | NULL  |                2 |
|  4 | Contact 3    | NULL          | contact3@fdsfsd.com    | NULL             | NULL             | Title3       | NULL  |                1 |
+----+--------------+---------------+------------------------+------------------+------------------+--------------+-------+------------------+
3 rows in set (0.00 sec)

mysql> select * from Address;
+----+-------------+---------+----------+-------+-----------+-----------------+------------------+---------------------+
| id | address     | city    | postcode | state | country   | addressClientId | addressContactId | addressEngagementId |
+----+-------------+---------+----------+-------+-----------+-----------------+------------------+---------------------+
|  1 | Address n.1 | Sydney  | 2000     | NSW   | Australia |               1 |             NULL |                NULL |
|  2 | Address n.2 | Adelaide| 2010     | NSW   | Australia |               2 |             NULL |                NULL |
| 19 | Address n.3 | Perth   | 2050     | NSW   | Australia |               1 |                4 |                NULL |
+----+-------------+---------+----------+-------+-----------+-----------------+------------------+---------------------+
3 rows in set (0.01 sec)

预期结果类似于

ContactName, CompanyName, City

Contact 1, Client1, Sydney
Contact 2, Client1, Adelaide
Contact 3, Client2, Perth

Answer 1

您可以按照下面的表顺序在这三个表中使用left join：

select cn.name as ContactName, cl.name as ClientName, ad.city
  from Contacts cn
  left join Clients cl on cl.id = cn.contactsClientId 
  left join Address ad on ad.addressClientId = cl.id and ad.addressContactId = cn.id
 order by ContactName;

考虑 Demo ，如果您具有演示中的数据，则可以得到所需的结果。

Answer 2

这是我想出的：

SELECT co.name, cl.name, IF(a1.id IS NULL, a2.city, a1.city) AS city
FROM Contacts co
JOIN Clients cl
     ON co.contactsClientId = cl.id
LEFT JOIN Address a1
     ON co.id = a1.addressContactId
LEFT JOIN Address a2
     ON cl.id = a2.addressClientId
     AND a1.id IS NULL
     AND a2.addressContactId IS NULL
ORDER BY co.name

但是请注意，您的Contact 1正在为Client1工作，该地址有两个地址。因此，在这种情况下，我添加了AND a2.addressContactId IS NULL行以仅选择一个未分配addressContactId的地址，但是根据您的实际数据，您可能仍然会重复（如果您有一个联系人在一家公司工作，而两个联系人没有addressContactId的地址）

用两个外键联接两次表

2 个答案: