sql - 多对多 SQL 查询

Question

我有三个表：Requests、Users 和 UserRequests。

请求： - ID - Submit_ID - Assign_ID - Completed_ID

用户： - ID - 用户名

UserRequest: - ID - Request_ID - User_ID

每个请求都有一个不同的用户，而不是提交、分配和完成的请求。我正在尝试以下输出：

请求.ID | 分配的用户名 | 提交的用户名 | 完成的用户名

我要这样做是错误的方式吗？

Answer 1

不明白 UserRequest 的目的，但您的预期输出如下：

SELECT
    r.ID,
    us.UserName,
    ua.UserName,
    uc.UserName
FROM
    Requests r
    JOIN Users us 
    ON r.Submit_ID = us.ID
    JOIN Users ua
    ON r.Assign_ID = ua.ID
    JOIN Users uc
    ON r.Complete_ID = uc.ID

Answer 2

您可以通过为 User 表为每个连接提供不同的别名来执行您的要求（多个一对多连接），如下所示：

SELECT  
    Requests.ID AS `Request.ID`,  
    AssignUser.UserName AS `Username of Assigned`,  
    SubmitUser.UserName AS `Username of Submitted`,  
    CompleteUser.UserName AS `Username of Completed`  
FROM  
    Requests  
JOIN User AS AssignUser ON  
    Requests.Assign_ID = AssignUser.ID  
JOIN User AS SubmitUser ON  
    Requests.Submit_ID = SubmitUser.ID  
JOIN User AS CompleteUser ON  
    Requests.Completed_ID = CompleteUser.ID  

Note that the back ticks are used above for specifying the column aliases. If you aren't using MySQL, you would probably have to use double-quotes for Oracle (or a different character if you are using a different kind of database).