Creating hierarchical menu in asp.net using SQL - asp.net

Question

I'm trying to use to create a hierarchical menu from a SQL data source in asp.net. I'm having trouble ordering my table so that I can easily create the menu in asp.net. There may be a better way to do it if anyone has any ideas...
I currently have a table that looks like this (made up sample data), there are only folders at the root and only folders can have folder Ids:
+-------------------+---------------------+-----------+-----------+
| Name_FolderorItem | Parent_Folder | Id_Folder | Menuplace |
+-------------------+---------------------+-----------+-----------+
| c FOLDER | ROOT | c_FOLDER | 1 |
| d FOLDER | j_FOLDER | d_FOLDER | 2 |
| a FOLDER | ROOT | a_FOLDER | 1 |
| j FOLDER | ROOT | j_FOLDER | 1 |
| f FOLDER | ROOT | f_FOLDER | 1 |
| r FOLDER | f_FOLDER | r_FOLDER | 2 |
| i FOLDER | d_FOLDER | i_FOLDER | 3 |
| a ITEM | j_FOLDER | | 2 |
| d ITEM | c_FOLDER | | 2 |
| z ITEM | f_FOLDER | | 2 |
| r ITEM | d_FOLDER | | 3 |
+-------------------+---------------------+-----------+-----------+
I'm thinking that If I order it to get this which is alphabetical on the first level then alphabetical on each deeper level:
+-------------------+---------------------+-----------+-----------+
| Name_FolderorItem | Parent_Folder | Id_Folder | Menuplace |
+-------------------+---------------------+-----------+-----------+
| a FOLDER | ROOT | a_FOLDER | 1 |
| c FOLDER | ROOT | c_FOLDER | 1 |
| d ITEM | c_FOLDER | | 2 |
| f FOLDER | ROOT | f_FOLDER | 1 |
| r FOLDER | f_FOLDER | r_FOLDER | 2 |
| z ITEM | f_FOLDER | | 2 |
| j FOLDER | ROOT | j_FOLDER | 1 |
| a ITEM | j_FOLDER | | 2 |
| d FOLDER | j_FOLDER | d_FOLDER | 2 |
| i FOLDER | d_FOLDER | i_FOLDER | 3 |
| r ITEM | d_FOLDER | | 3 |
+-------------------+---------------------+-----------+-----------+
Then I can use a listview to get this menu structure:
a FOLDER
c FOLDER
- d ITEM
f FOLDER
- r FOLDER (r FOLDER is located in f folder)
- z ITEM
j FOLDER
- a item
- d FOLDER
- - i FOLDER
- - r ITEM
I can't seem to figure out the required SQL to take a folder and then prioritise what is inside of it instead of the other folders/items on that level.
If you have any ideas on the SQL statements that would allow for this ordering I would appreciate it, thanks in advance
EDIT:
Here is the Query that I'm now using, thanks for the help
SELECT *
FROM table
START WITH Parent_Folder LIKE 'ROOT'
CONNECT BY PRIOR Id_Folder LIKE Parent_folder;

Answer 1

This is basically the problem of storing an ordered tree in a database. I have tried a few approaches in the past and would recommend this one:
-----------------------------------
| NodeID | ParentID | Order |
-----------------------------------
| 1 | -1 | 1 |
| 2 | 1 | 1 |
| 3 | 1 | 2 |
| 4 | 3 | 1 |
-----------------------------------
The GIST is each row corresponds to a node in the tree (or a menu item in your case), and it has a parentID that points to the parent. The root node either has null or -1 has its parentID since it does not have one. Now, to get all the nodes that are directly under the root, you'd use
SELECT * FROM table WHERE ParentID = -1 ORDER BY [Order]
[Order] tells you how to order the nodes in a directly under a specific parent node. (OK, it's not so good to name a column using a SQL keyword, but let's forget that for the moment). So in my example you can see both "2" and "3" are under the root, but "2" comes first. To select the nodes directly under "3" (its children in other words), you'd use
SELECT * FROM table WHERE ParentID = 3 ORDER BY [Order]
The advantage of this approach is it's easy to query the menu. The disadvantage is when you update, you have to make sure nodes that share the same ParentID (in other words under the same parent) do not share the name [Order].
Alternative approach
An alternative approach is to store the tree as something like a link list:
-----------------------------------
|NodeID | ParentID | PrevID | (optionally you can store NextID as well)
-----------------------------------
| 1 | -1 | -1 |
| 2 | 1 | -1 |
| 3 | 1 | 2 |
| 4 | 3 | -1 |
-----------------------------------
If your tree is very large and you need to quickly browse the sibling nodes, this approach would offer a better performance.