Question

Assuming I have a table like this one:

CREATE TABLE user_delegates (
    [id] INT IDENTITY(1,1) NOT NULL,
    [user_from] VARCHAR(10) NOT NULL,
    [user_to] VARCHAR(10) NOT NULL,
    CONSTRAINT [PK_user_delegates] PRIMARY KEY CLUSTERED ([id] ASC),
    CONSTRAINT [UK_user_delegates] UNIQUE ([user_from] ASC)
)

So an user A has to right to delegate her system access to another user B. When she does that, she won’t be able to access the system anymore – user B will have to “break” that delegation before she is able to use the system again…

BUT also consider that, if user B delegates access to user C, user C will also start impersonating user A, and so on.

(I know this seems to be a security nightmare – please let’s just forget about that, OK? :-))

Also consider those records:

INSERT INTO user_delegates([user_from], [user_to]) values ('ANTHONY', 'JOHN')
INSERT INTO user_delegates([user_from], [user_to]) values ('JOHN', 'JOHN')
INSERT INTO user_delegates([user_from], [user_to]) values ('KARL', 'JOSHUA')
INSERT INTO user_delegates([user_from], [user_to]) values ('JOSHUA', 'PIOTR')
INSERT INTO user_delegates([user_from], [user_to]) values ('PIOTR', 'HANS')

So what I need is finding the last (which means the active) delegation for each user.

I have come to a solution that I’ve decided to not show here (unless everybody ignores me, which is always a possibility). All I can say is that it is a somewhat long answer, and it surely seems like using a cannon to kill a flea…

But how would you do that? Consider any relevant SQL Server extension available, and notice we’re looking for an answer that is both elegant and with a good performance…

BTW, this is the expected result set:

id          user_from  user_to   
----------- ---------- ----------
1           ANTHONY    JOHN      
2           JOHN       JOHN      
3           KARL       HANS      
4           JOSHUA     HANS      
5           PIOTR      HANS      

(5 row(s) affected)

And thanks in advance!

Answer 1

WITH    q (user_initial, user_from, user_to, link) AS
        (
        SELECT  user_id, user_id, user_id, link
        FROM    users
        UNION ALL
        SELECT  user_initial, q.user_to, ud.user_to, link + 1
        FROM    q
        JOIN    user_delegates ud
        ON      ud.user_from = q.user_to
        )
SELECT  *
FROM    (
        SELECT  *, ROW_NUMBER() OVER (PARTITION BY user_initial ORDER BY link DESC) rn
        FROM    q
        )
WHERE   rn = 1