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:

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

Now I need an SQL command that can display all items belong to a category and its subcategories. For example, if I choose CategoryID=2 I will get:
iPhone 5
iPad 2
iPhone 4
iPad Air

If I choose CategoryID=3 I will get:
Motorola Droid
Samsung S1
Samsung S3

If I choose CategoryID=6 I will get:
Dell Laptop

And if I choose CategoryID=1 I will get all items. I am thinking of using Common Table Expression but can not figure out how to do it.