Need Help Extending A Query

I need some help forming a query

I have 3 Tables...

Table1: Users
Fields: UserID Int
Username Varchar(50)

Table2: User_Categories
Fields: User_CatID Int
User_ID Int

Table3: Categories
Fields: CatID Int
Catname Varchar(100)


Now consider Table 3 has the following values

CatID | Catname
1 | cat1
2 | cat2
3 | cat3
4 | cat4
5 | cat5
6 | cat6
7 | cat7
8 | cat8
9 | cat9


Now I need to select users who fall into the categories with CatID 5, 1, 3. Also users can fall into more than one category.

The users should be sorted by the 1st the users who fall under 5, then users who fall under 1 and then the users who fall under 3 and then finally by the userID in the descending order.


SELECT userid, username
FROM Users, User_Categories
WHERE userid=user_catid AND user_catID IN (5, 1, 3)
ORDER BY userid DESC;

How do I extend this to get the above needed result? Any help will be highly appreciated.