使用列值获取表连接

Question

如何使用列值连接表？

我有三个表，如下所示：

• 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，将关于列值的表连接：

1. 如果user_type = admin那么它应该与admin_table加入。

2. 如果user_type = manager那么它应该与manager_table加入。

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。

---

编辑：

我想从sql查询中获取数据而不是从服务器端编码。

我从这里尝试了这个查询，即Winfred的想法（已回答）

但是，我无法理解。

msg_by_usertype是基于列的，值manager然后应该选择manager_table ，如果是admin，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小提琴 ，看看它是如何工作的。 （它只有在你不能拥有像经理一样具有相同身份的管理员时才有效。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，你想要这样的结果吗？

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，你可以使用它

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