使用列值獲取表連接

Question

如何使用列值連接表？

我有三個表，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查詢獲得如下所示的所需輸出。 即，滿足以下條件時，將關於列值的表連接：

1. 如果user_type = admin那么它應該與admin_table加入。

2. 如果user_type = manager那么它應該與manager_table加入。

期望的輸出：

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

即根據列值獲取join sql查詢。

---

編輯：

我想從sql查詢中獲取數據而不是從服務器端編碼。

我從這里嘗試了這個查詢，即Winfred的想法（已回答）

但是，我無法理解。

msg_by_usertype是基於列的，值manager然后應該選擇manager_table ，如果是admin，則選擇admin_table

Answer 1

據我了解你的問題，你可以試試這個：

    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  

這是一個SQL小提琴 ，看看它是如何工作的。 （它只有在你不能擁有像經理一樣具有相同身份的管理員時才有效。在兩個表中，Id應該是唯一的。）

Answer 2

請使用以下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

如果我理解你的問題，你想要這樣的結果嗎？

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

如果是這樣，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