Interesting tree/hierarchical data structure problem

Question

Colleges have different ways of organizing their departments. Some schools go School -> Term -> Department. Others have steps in between, with the longe

Answer 1

I would develop this in a very flexible manner and what seems to mean to be the simplest as well:

There should only be one table, lets call it the category_nodes:

-- possible content, of this could be stored in another table and create a
-- 1:N -> category:content relationship
drop table if exists category_nodes;
create table category_nodes (
  category_node_id int(11) default null auto_increment,
  parent_id int(11) not null default 1,
  name varchar(256),
  primary key(category_node_id)
);
-- set the first 2 records:
insert into category_nodes (parent_id, name) values( -1, 'root' );
insert into category_nodes (parent_id, name) values( -1, 'uncategorized' );

So each record in the table has a unique id, a parent id, and a name.

Now after the first 2 inserts: in category_nodes where the category_node_id is 0 is the root node (the parent of all nodes no matter how many degres away. The second is just for a little helper, set an uncategorized node at the category_node_id = 1 which is also the defalt value of parent_id when inserting into the table.

Now imagining the root categories are School, Term, and Dept you would:

insert into category_nodes ( parent_id, name ) values ( 0, 'School' );
insert into category_nodes ( parent_id, name ) values ( 0, 'Term' );
insert into category_nodes ( parent_id, name ) values ( 0, 'Dept' );

Then to get all the root categories:

select * from category_nodes where parent_id = 0;

Now imagining a more complex schema:

-- School -> Division -> Department
-- CatX -> CatY
insert into category_nodes ( parent_id, name ) values ( 0, 'School' ); -- imaging gets pkey = 2 
insert into category_nodes ( parent_id, name ) values ( 2, 'Division' ); -- imaging gets pkey = 3
insert into category_nodes ( parent_id, name ) values ( 3, 'Dept' );
--
insert into category_nodes ( parent_id, name ) values ( 0, 'CatX' ); -- 5
insert into category_nodes ( parent_id, name ) values ( 5, 'CatY' );

Now to get all the subcategories of School for example:

select * from category_nodes where parent_id = 2;
-- or even
select * from category_nodes where parent_id in ( select category_node_id from category_nodes 
    where name = 'School'
);

And so on. Thanks to a default = 1 with the parent_id, inserting into the 'uncategorized' category become simple:

Cheers