Question

好的查询从三个表中获取信息以获取帐户详细信息，从管理员，员工和客户端问题是我的，因为有人只存在于Admin_Agency，Agency_Employee或Client表中我最终得到了一堆列包含NULLS，见下文。

在这里，我想以某种方式组合以下列，以便没有NULL列：

Client.First_Name，Admin_Agency.Admin_First_Name和Admin_Agency.Admin_First_Name 进入First_Name

Client.Last_Name，Admin_Agency.Admin_Last_Name和Admin_Agency.Admin_Last_Name 进入Last_Name

最后所有列都命名为“Profile_Pic”。

可以这样做吗？

+--------------------+---------------------+------------+------------------+-----------------+-------------+------------+-----------+-------------+------------+-----------+-------------+
| User_Comment       | Time_Stamp          | Account_ID | Admin_First_Name | Admin_Last_Name | Profile_Pic | First_Name | Last_Name | Profile_Pic | First_Name | Last_Name | Profile_Pic |
+--------------------+---------------------+------------+------------------+-----------------+-------------+------------+-----------+-------------+------------+-----------+-------------+
| Lorem ipsum dolor  | 2012-03-21 23:40:15 |         18 | Fredric          | Beskoski        | 002.jpg     | NULL       | NULL      | NULL        | NULL       | NULL      | NULL        |
+--------------------+---------------------+------------+------------------+-----------------+-------------+------------+-----------+-------------+------------+-----------+-------------+
| Lorem ipsum dolor  | 2012-04-21 23:40:05 |         20 | NULL             | NULL            | NULL        | Alan       | James     | 032.jpg     | NULL       | NULL      | NULL        |
+--------------------+---------------------+------------+------------------+-----------------+-------------+------------+-----------+-------------+------------+-----------+-------------+
| Lorem ipsum dolor  | 2012-05-24 21:40:12 |         21 | NULL             | NULL            | NULL        | NULL       | NULL      | NULL        | David      | Calson    | 044.jpg     |
+--------------------+---------------------+------------+------------------+-----------------+-------------+------------+-----------+-------------+------------+-----------+-------------+

请注意，Admin_First_Name，Admin_Last_Name和Profile_Pic已填充，但其余为NULL。

查询：

SELECT 
    Comments.User_Comment, Comments.Time_Stamp, Account.Account_ID,
    Admin_Agency.Admin_First_Name, Admin_Agency.Admin_Last_Name, Admin_Agency.Profile_Pic,
    Agency_Employee.First_Name, Agency_Employee.Last_Name, Agency_Employee.Profile_Pic,
    Client.First_Name, Client.Last_Name, Client.Profile_Pic

FROM Comments
    LEFT JOIN Account ON (Account.Account_ID = Comments.Account_ID_FK4)
    LEFT JOIN Admin_Agency ON (Account.Account_ID = Admin_Agency.Agency_ID)
    LEFT JOIN Agency_Employee ON (Account.Account_ID = Agency_Employee.Employee_ID)
    LEFT JOIN Client ON (Account.Account_ID = Client.Client_ID)

WHERE Comments.Design_ID_FK = 5

感谢您的帮助！

Answer 1

左外连接或右外连接可以为空提供。

内部联接可能会减少行数，但没有一行会引入“join”中的任何空值。

Answer 2

您应首先设置优先级以检查哪一个（来自客户端或管理员的值）。例如，如果要检查第一个值是否存在于客户端，如：

select if(Client.First_Name is Null,Admin_Agency.Admin_First_Name,Client.First_Name) 
as first_name ,...   //Same for other columns
From ...
Joins....
Where ...

合并列加入？

2 个答案: