sql - 为了创建我的模型，我需要执行一个非常困难的查询

Question

所以在我的项目中，我需要访问一个名为“日期”的表。表“日期”包含以下内容:

ClassID | TeacherID | StudentID | DateInfo | Status | DateID

对于除 DateID 之外的任何 ID，我需要创建一个连接到日期模型的模型。 另一个重要的信息是我有一个名为“用户”的表和 2 个派生自“用户”、“学生”和“教师”的表。

My Problem: because I need both the student and the teacher in the same record, I need to request from the DB the user's information about both of them in the same query. I can not do that because of i don't know how can I define the student's information different from the teacher information.

用户表:

UserID | Name | LastName | Password | Status | Role 

教师表:

TeacherID | ClassID

学生表:

StudentID | YearAndClass

我正在尝试创建的查询:

SELECT
IF(Users.Role == 0(Student Role))
  Users.Name as sName, ....
ELSE IF(Users.Role == 1(Teacher Role))
  Users.Name as tName, ....  
FROM Dates, Users, Students, Teachers

我知道这行不通，但我不确定我还能如何描述它。

提前致谢!

Answer 1

根据我对你的问题的评论，我仍然认为你应该回去稍微修改你的结构，你似乎大部分都在那里，但最好在使用它之前让任何过程的基础正确。

就是说，据我了解，您希望获得 1 个列表中的用户名，但可以按其角色区分。以下应该有效。

我添加了 UserIDs 和 Role 字段，以便您可以根据您的数据检查它们，看看它是否按预期工作。

    SELECT
     CASE   WHEN d.Role = 'Teacher Role' THEN ut.UserID
            WHEN d.Role = 'Student Role' THEN us.UserID
            ELSE ''
        END AS UserID
    ,CASE   WHEN d.Role = 'Teacher Role' THEN ut.Name
            WHEN d.Role = 'Student Role' THEN us.Name
            ELSE ''
        END AS UserName
    ,d.Role
    FROM Dates d
    LEFT JOIN Users ut ON d.TeacherID = ut.UserID
    INNER JOIN Teachers t ON ut.UserID = t.TeacherID
    LEFT JOIN Users us ON d.StudentID = us.UserID
    INNER JOIN Students s ON us.UserID = s.StudentID

或者可能是下面的(未经测试)；

    SELECT
     CASE   WHEN d.Role = 'Teacher Role' THEN ut.UserID
            WHEN d.Role = 'Student Role' THEN us.UserID
            ELSE ''
        END AS UserID
    ,CASE   WHEN d.Role = 'Teacher Role' THEN ut.Name
            WHEN d.Role = 'Student Role' THEN us.Name
            ELSE ''
        END AS UserName
    ,d.Role
    FROM Dates d
    LEFT JOIN Users ut ON d.TeacherID = ut.UserID AND ut.UserID IN (SELECT DISTINCT TeacherID FROM Teachers)
    LEFT JOIN Users us ON d.StudentID = us.UserID AND us.UserID IN (SELECT DISTINCT StudentID FROM Students)