Database tree structure functionality

[Sh_oK]

I have a database that calculates costs of a company using a managerial accountancy method (activity based costing).
The database uses 6 tables with information about the departments (cost centers), activity of the departments (process steps) etc. Using a few query’s one can find out which are the total costs per product.

Now, I should start to implement a functionality that must be able to arrange all departments in a hierarchy, i.e. in a tree structure, and store it in a table of the database. The task is not to program such functionality, but to research and find a software library where this problem is already solved.
If anyone has encountered this problem before or if you have some advice that could help me, I would be thankful.

Thank you!