复杂的SQL表关系？

Question

I have a problem about database table and query of it. Let me tell what this is about. It is about project-manager-employee relationship. Now Let me tell in statements what their relationship...

Manager = Manager is a project leader or could be manager of MORE than ONE project.

Employee = (s)He is a person who only works on project or projectS which is managed by Manager.

Project = Project is a project which managed by only ONE Manager.

Employee - Manager Relationship

Manager is also a employee in company right? According to this I want their relationship like -> one Employee can be managed by more than ONE Manager.
All managers are in the same title I mean there is no manager of a manager. There is only manager(s) for an employee. For example 5 employees can be managed by a Manager.

Employee - Project Relationship

Employee can have a project that make sense, also employee can have more than ONE project that's make sense too. So project can have more than ONE employee too. (reminder: Project can have more than ONE Employee, BUT can NOT be managed by more than ONE Manager).

Manager - Project Relationship

Manager can have more than ONE project. BUT Project can NOT be managed by more than ONE Manager.

PS: According to project, same employee in different projects CAN BE managed by more than one manager See my example below...

Android App PROJECT's Manager ----> Patrick

Android App PROJECT's Employees -----> John, Susan, Megan , Joe.

Another Project...

IOS App PROJECT's Manager ----> Ryan

IOS App PROJECT's Employees ----> Claire, Walter, Robert, Megan

Now let me show my tables for that... I can't really know what should I do...

In that table I tried to do that "manager_id(FK)" refer to "employee_id(PK)" because Manager is a employee in company.

And I have "manager_id(FK)" in Project table to refers "manager_id" in employee table.

And which is proj_employee table I didn't clearly understand. I found this way in examples from internet. That way we call them in query.

NOW let me post my queries I tried but never get a solution...

        select
    e.name as employee_name,
    pm.name as manager_name
from Proj_Employee as pe
    left outer join Employee as e on e.employee_id = pe.emp_id
    left outer join Project as p on p.project_id = pe.project_id
    left outer join Employee as pm on pm.employee_id = p.manager_id
where pe.project_id = <project id>

I would like to ask this... I want a query which shows me EmployeeName, ManagerName OF choosen project. And if my table is wrong Could you guide me or tell me how can I do that?

I have knowledge but couldn't figure it out how can I make this work.

Answer 1

I think I see the problem (finally).

You said, "And I have "manager_id(FK)" in Project table to refers "manager_id" in employee table."

I think that may be the problem. I think you want manager_id in the project table to be a foreign key to the employee_id column in the employee table, NOT the manager_id column. But you've got it right in your query.

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SQL Fiddle demo here: http://www.sqlfiddle.com/#!2/e31fb/1

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Not really related to your question, but I just want to take the opportunity to point out that this is a case where having the primary key column in each table named id helps us make "wrong" SQL look like "wrong" SQL. "Making Wrong Code Look Wrong" - Joel Spolsky

If we expect that:

primary keys will be of the form: id

foreign keys will of the form: parent_table_id , or parent_role_id

the vast majority of joins will be from foreign key to primary key

then we expect that most joins will be of the form

child.parent_id = parent.id

and that foreign key definitions will be of the form

FK_child_parent_or_role (parent_or_role_id) REFERENCES parent (id)

This makes a foreign key definition like this:

FK_project_manager (manager_id) REFERENCES employee (manager_id)

look "wrong" to us. We expect to see the reference to the id column.

Answer 2

You can get a list of (project_name, manager_name, employee_name) with something like this:

SELECT p.project_name
     , e.name as employee_name
     , m.name as manager_name
FROM project p
INNER JOIN employee m ON p.manager_id = m.employee_id
LEFT JOIN proj_employee pe ON p.project_id = pe.project_id
LEFT JOIN employees e ON pe.emp_id = e.employee_id;

You will get something like this for two projects, one having no employees:

 project_name  | employee_name | manager_name 
---------------+---------------+--------------
 Stack O       | Alice         | Eve
 Stack O       | Bob           | Eve
 Lemon Squeezy |               | Alice

Answer 3

There is no reason to have outer joins that I can see.

select p.Project_Name, m.name as Manager, e.name as Employee
from Project P
join Employee M on m.employee_id = p.manager_id
join project_employee pe on pe.project_id = p.project_id
join Employee E on e.employee_id = pe.employee_id