问题
I'm trying to draw an ER diagram describing the following:
-"Department" employs "Employees"
-Some "Employees" are "Special" and have more attributes
-Some of the "Employees" ("Special" and non special) are "Managers"
-"Managers" manage "Departments"
So, to convey this I have:
Department ------(employs)------- Employee-----<--------Special
| |
|-----------(manages)----------
From my understanding, I cannot have 2 relationships between 2 entities. How do I deal with this situation then?
回答1:
There is no reason you can't have multiple relationships between the same two entities, your understanding is wrong in this respect.
You have to be certain, of course, that the relationships capture different information and that you haven't, say, mistaken the two aspects of a single relationship as two different relationships. Your example seems to be a good candidate for having two relationships between the two entities.
回答2:
Hi mate watch this diagram, hope it helps
回答3:
Better approach is to add a bit field in Employee table IsManager, so if he is a manager, that means that he is a manager on department he works.
回答4:
Your relationship for employs and manages are going to be two linking tables. e.g. Consider the tables and fields as below:
Department
DepartmentID
DepartmentName
Employee
EmployeeID
EmployeeName
SpecialEmployee
EmployeeID
Attr1
Attr2
Now you can go on and link Employee and Department as such:
DepartmentEmploys
DepartmentEmploysID
DepartmentID
EmployeeID
ManagesDepartment
ManagesDepartmentID
EmployeeID
DepartmentID
Now your relationships do not overlap or interfere. What seems to be drawn in your diagram is correct. There can be more than one relationships between two entities.
回答5:
There should be only one class of employee If you have multiple categories of the employee then you can make child classes of Employee Class.
Parent: Employee
1st child: Special Employee
2nd child: Normal Employee
来源：https://stackoverflow.com/questions/14542763/2-relationships-between-2-entities-in-er-diagram