How to map?

I have a problem about OR mapping.

The situation is:
Branches have departments, and each department of branch could
have groups. And sometimes, groups could directly belong to branch but not
department.

Suppose I have the following classes,

Class Branch
-------------
int branchID
String name
Department[] dept
Group[] group

Class Department
----------------
int deptID
String deptName

Class Group
----------------
int groupID
String groupName

How to set dept and group in branch table?
Are these three classes could satisfy the situation?

Thanks for advise.

what ORM tool are you using ?
k

What's the real significance of the department/branch/group distinction? Do you need to store very different data for each? Are their lots of business rules regarding their behavior?

I don't use any ORM tool.
I just want to know how to map the classes to the relational tables.

I think the distinction of the department/branch/group is not very obvious.
Because they all have names, id ...etc.
But their relationship is like a tree. For instance,
branch has some departments, and departments have groups,
and sometimes branch has groups.

And their bahaviors are not all the same, just like different employees in different departments don't have the same behaviors.

Here's the reason I ask: One approach is to just use a single class (and corresponding table), called, say, OrganizationalUnit. There could also be an OUType class, essentially an enumeration. Then label the OUs with their types, and enforce the requirements about fits where in the tree as business rules. One reason for doing it this way is that the rules may change over time.

Thank you. I will try the method you said.