SQL One-to-many or NONE to many? - sql-server

Question

I have a one-to-many join. Works flawlessly, and quickly. The one-to-many may have zero records in the many table. If this is the case, I want to join it to ALL records in the many table.
USERS
-----
UserID | Name
CATEGORIES
------------------
CategoryID | CategoryName
USER_CATEGORIES
---------------
UserID | CategoryID
IF there are NO categories assigned to the user, I would like to join ALL categories. The reasoning behind this is, some users may manage all categories. If a category is added, it's automatically assigned to those users.
1 | Michael
2 | Bob
100 | Billing
101 | Email
102 | Technical
1 | 101
I would like the result to be:
1 | Michael | 101 | Email
2 | Bob | 100 | Billing
2 | Bob | 101 | Email
2 | Bob | 102 | Technical
So far, the way it works is:
DECLARE #USERS TABLE (UserID INT, UserName VARCHAR(10));
DECLARE #USER_CATEGORIES TABLE (UserID INT, CategoryID INT);
DECLARE #CATEGORIES TABLE (CategoryID INT, CategoryName VARCHAR(10));
INSERT INTO #USERS (UserID, UserName)
(
SELECT 1, 'Michael' UNION ALL
SELECT 2, 'Bob'
)
INSERT INTO #CATEGORIES (CategoryID, CategoryName)
(
SELECT 100, 'Billing' UNION ALL
SELECT 101, 'Email' UNION ALL
SELECT 102, 'Technical'
)
INSERT INTO #USER_CATEGORIES (UserID, CategoryID)
(
SELECT 1, 101
)
SELECT
U.UserID
, U.UserName
, C2.CategoryID
, C2.CategoryName
FROM
#USERS U LEFT JOIN
#USER_CATEGORIES UC ON U.UserID = UC.UserID LEFT JOIN
#CATEGORIES C ON UC.CategoryID = C.CategoryID LEFT JOIN
#CATEGORIES C2 ON C.CategoryID = C2.CategoryID OR (C.CategoryID IS NULL)
Is this the correct way to handle this? It seems to be a little too much overhead when I have many users/category combinations.

Answer 1

You can remove one of the joins to #CATEGORIES:
SELECT
U.UserID
, U.UserName
, C2.CategoryID
, C2.CategoryName
FROM #USERS U
LEFT JOIN #USER_CATEGORIES UC
ON U.UserID = UC.UserID
LEFT JOIN #CATEGORIES C2
ON UC.CategoryID = C2.CategoryID
OR (UC.CategoryID IS NULL)

Answer 2

Erm, if you were querying for one category or one user, then while I find it aesthetically offensive, okay.
For all users and categories? Not for me that, no
Can't say which way you should go. because I haven't a clue why you want this data
But me I'd either return say 'ALL' the result of the inner join was null, or I'd just use it returning no records for that user, to trigger a select * from categories if I needed it and hadn't already cached it.