SQL: Order by rowID and then by column based on the rowid

Question

I have the following 'departments' table:

+-------+-------------+-------------------+
| rowid | department  | parent_department |
+-------+-------------+-------------------+
| 10    | Main Office | NULL              |
+-------+-------------+-------------------+
| 11    | Back Office | 10                |
+-------+-------------+-------------------+
| 12    | Commercial  | NULL              |
+-------+-------------+-------------------+
| 13    | Outdoor     | NULL              |
+-------+-------------+-------------------+
| 14    | Beach       | 13                |
+-------+-------------+-------------------+
| 15    | Gardening   | 13                |
+-------+-------------+-------------------+
| 16    | Accounting  | 10                |
+-------+-------------+-------------------+

and I would like to order it based on rowid and parent_department as such:

+-------+-------------+-------------------+
| rowid | department  | parent_department |
+-------+-------------+-------------------+
| 10    | Main Office | NULL              |
+-------+-------------+-------------------+
| 11    | Back Office | 10                |
+-------+-------------+-------------------+
| 16    | Accounting  | 10                |
+-------+-------------+-------------------+
| 13    | Outdoor     | NULL              |
+-------+-------------+-------------------+
| 14    | Beach       | 13                |
+-------+-------------+-------------------+
| 15    | Gardening   | 13                |
+-------+-------------+-------------------+
| 12    | Commercial  | NULL              |
+-------+-------------+-------------------+

So that for each rowid , find if 'parents_departments' exists and show order it as the following.

Please note that I have not designed the current table and sadly I do not have the permission to change it

Answer 1

Try this:

SELECT rowid, department, parent_department
  FROM departments
 ORDER BY IFNULL(parent_department, rowid), rowid;