Question

This might be simple one but I am finding it very difficult to work around it.

I have got 3 tables like; Employee, manager, supervisor in here manager and supervisor is the specialization of the table employee. Now I have got another table say Project which should have 1 manager and 3 supervisor.

How do I put this in my mysql database?

As Employee has Employeeid, name, position and manager will have employeeid as pk and fk sme with supervisor which will have employeeid as pk and fk. How do I put the data in manager and supervisor table? Do I use a view?

How would mysql know which is manager and which is supervisor?

Answer 1

How to connect your tables:

You can make an additional table ProjectMember with the following columns:

employeeId
projectId
role

Where role can be ‘manager’ or ‘supervisor’.

So, if these are your Employees:

employeeId    name
1             Mark
2             Steve
3             Simon
4             Stan

And this is your Project:

projectId    name
1            Project X 

Then the ProjectMember table can have the following rows:

employeeId projectId role
1          1         manager
2          1         supervisor
3          1         supervisor
4          1         supervisor

To get the names of all members of Project X you can then use the following query:

SELECT emp.name AS name, pm.role AS role
FROM ProjectMember pm
JOIN Employee emp
ON pm.employeeId = emp.employeeId
WHERE pm.projectId = 1

This will return:

name   role
Mark   manager
Steve  supervisor
Simon  supervisor
Stan   supervisor

Adding specializations

If there is any specific data for a Manager or a Supervisor as specializations of Employee, then you can retrieve the additional data as follows.

Suppose Mark is a Manager, then this can be a row in Manager:

employeeId clearanceLevel
1          10

EDIT: To store a Manager, you first store an Employee and get its (probably generated) employeeId. How to get a generated id depends on the database system you are using (see this on MySQL). Then you store a row in Manager with that same employeeId.

To get this additional information, while you do not really know who is in the Manager table on beforehand, you can use a LEFT JOIN to retrieve data from optional additional tables (they may or may not be there, but you still want to get the basic queried rows):

SELECT emp.name AS name, pm.role AS role, mgr.clearanceLevel AS clearanceLevel
FROM ProjectMember pm
JOIN Employee emp
ON pm.employeeId = emp.employeeId
LEFT JOIN Manager mgr
ON mgr.employeeId = emp.employeeId
WHERE pm.projectId = 1

This will return:

name   role       clearanceLevel
Mark   manager    10
Steve  supervisor NULL
Simon  supervisor NULL
Stan   supervisor NULL