Sql 获取具有列值的表联接

如何使用列值联接表

我有以下三个表格：

• 消息\u表

  -----------------------------------------------------------------------------
  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_id    | admin_name
  -------------------------—
  001         | Super Admin
  

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

• 管理表

  -----------------------------------------------------------------------------
  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_id    | admin_name
  -------------------------—
  001         | Super Admin
  

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

• manager\u表

  -----------------------------------------------------------------------------
  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_id    | admin_name
  -------------------------—
  001         | Super Admin
  

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

如何使用SQL查询获得所需的输出，如下所示。即。 满足以下条件时，根据列值联接表：

如果

user\u type=admin

，那么它应该与

admin\u表

连接

如果

user\u type=manager

，那么它应该与

manager\u表

连接

所需输出：

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

即根据列值获取联接sql查询

---

编辑：

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

我想从sql查询中获取数据，而不是从服务器端编码中获取数据

我试着从Winfred的想法（已回答）提出这个问题

然而，我无法理解

---

msg\u by\u usertype

是基于列的，其中值

manager

则应选择

manager\u table

，如果是admin to

admin\u table

，据我所知，您可以尝试以下方法：

    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  

---

下面是一个例子，看看它是如何工作的。（只有当您的管理员不能像经理一样拥有相同的id时，它才起作用。id在两个表中都应该是唯一的。）

请使用下面的SQL语句

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 

---

如果我没弄错你的问题，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 

---

您要检索哪些列？此外，这些表之间的关系如何？示例消息表和管理表？他们的链接栏是什么？我编辑了这个问题，请看一看。