Select all items in a category and its sub categories

I have a Categories table and Items table like these:

Categories

CategoryID 	CategoryName	CategoryLevel	OrdinalNumber
1		All 			0		1
2		iOS			1		2
3		Android			1		5
4		Sub iOS 1		2		3
5		Sub Andoid 1		2		6
6		Others			1		8
7		Sub Andoid 2		2		7
8		Sub iOS 2		2		4

Items

ItemID		ItemName	CategoryID
1		iPhone 5		4
2		Motorola Droid	        5
3		iPad 2		        8
4		Samsung S1	        7
5		iPhone 4		4
6		iPad Air		8
7		Samsung S3	        7
8		Dell Laptop	        6

As you can see, they can be join by CategoryID. The Categories table, if sorted by OrdinalNumber and CategoryLevel, has tree-view display:

All
  iOS
     Sub iOS 1
     Sub iOS 2
  Android
     Sub Andoid 1
     Sub Andoid 2
  Others