6

如何使用列值连接表？

我有如下列出的三个表：

  • 消息表

    -----------------------------------------------------------------------------
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

  • 管理员表

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

  • manager_table

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

如何使用 SQL 查询获得所需的输出，如下所示。即当满足以下条件时，关于列值连接表：

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

  2. 如果user_type = manager那么它应该加入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。

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，manager那么它应该选择的值在哪里manager_table，如果它是 admin 到admin_table

4

3 回答 3

5

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，看看它是如何工作的。（仅当您不能拥有像经理一样具有相同 ID 的管理员时才有效。ID 在两个表中应该是唯一的。）

于 2012-09-13T09:51:41.450 回答
3

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
于 2012-09-13T10:48:40.933 回答
2

如果我正确理解你的问题，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
于 2012-09-13T10:52:17.553 回答