Question

如何使用列值连接表？

我有三个表格如下：

messages_table

-----------------------------------------------------------------------------
msg_id  | msg_sub   | msg_to    | to_user_type  | msg_from  | from_user_type
-----------------------------------------------------------------------------
001     | test      | 88        | manager       | 42        | admin
002     | test2     | 88        | manager       | 94        | manager

admin_table

-------------------------—
admin_id    | admin_name
-------------------------—
001         | Super Admin

manager_table

---------------------------
manager_id  | manager_name
---------------------------
88          | Mandela
94          | Kristen

如何使用SQL查询获得如下所示的所需输出。即满足以下条件时，连接有关列值的表：

如果user_type = admin则应加入admin_table。
如果user_type = manager则应加入manager_table。

期望的输出：

-----------------------------------------------------
msg_id  | msg_sub   | msg_to_name   | msg_from_name
-----------------------------------------------------
001     | test      | Mandela       | Super Admin
002     | test2     | Mandela       | Kristen

即。根据列值获取join sql查询。

修改

我想从sql查询中获取数据，而不是从服务器端编码。

我从here尝试了这个查询，即Winfred的想法（已回答）

然而，我无法理解。

msg_by_usertype是基于列的，其中值manager则应选择manager_table，如果是管理员，则选择admin_table

Answer 1

据我了解你的问题，你可以试试这个：

    SELECT msg_id,  
     msg_body,  
     usersBy.userName AS msg_by, 
     usersTo.userName AS msg_to,  
     msg_by_usertype
    FROM messages
    INNER JOIN 
    (SELECT admin_id As id, admin_name as userName
      FROM admin_table         
     UNION         
     SELECT manager_id As id, manager_name as userName
      FROM manager_table ) usersTo ON msg_to = usersTo.id  

     INNER JOIN 
    (SELECT admin_id As id, admin_name as userName
      FROM admin_table        
     UNION         
     SELECT manager_id As id, manager_name as userName
      FROM manager_table ) usersBy ON msg_by = usersBy.id

这是一个SQL Fiddle，看看它是如何运作的。（它只适用于你不能拥有像管理员一样具有相同身份的管理员。在两个表中，Id应该是唯一的。）

Answer 2

请使用以下SQL

SELECT msg_id,  
     msg_body,  
     usersBy.userName AS msg_by, 
     usersTo.userName AS msg_to,  
     msg_by_usertype
FROM messages
INNER JOIN 
    (SELECT admin_id As id, admin_name as userName,'admin' as usertype
      FROM admin_table         
     UNION         
     SELECT manager_id As id, manager_name as userName,'manager' as usertype
      FROM manager_table ) usersTo 
      ON msg_to = usersTo.id  and  msg_by_usertype = usersTo.usertype

Answer 3

如果我理解你的问题，你想得到这样的结果吗？

MSG_ID      MSG_BODY    MSG_TO      BY           MSG_BY_USERTYPE 
----------  ----------  ----------  -----------  --------------- 
001         test        adm1        managone     manager         
002         sadff       adm1        adm3?        admin

如果是这样，你可以使用这个

SELECT MSG_ID, MSG_BODY, MSG_TO,                                        
     CASE                                                               
         WHEN MSG_BY_USERTYPE = 'admin' THEN COALESCE(                  
              (SELECT ADMIN_NAME FROM ADMIN_TABLE                       
               WHERE  MSG_BY = ADMIN_ID), RTRIM(MSG_BY) CONCAT '?')     
         WHEN MSG_BY_USERTYPE = 'manager' THEN COALESCE(                
              (SELECT MANAGER_NAME FROM MANAGER_TABLE                   
               WHERE  MSG_BY = MANAGER_ID), RTRIM(MSG_BY) CONCAT '?')   
         ELSE ' '                                                       
     END AS BY,                                                         
 MSG_BY_USERTYPE                                                        
 FROM MESSAGES

使用列值获取表连接

3 个答案: