Question

I have table:

id | uid | action
-----------------
 1 |  1  |   2
 2 |  3  |   1
 3 |  2  |   2
 4 |  3  |   2
 5 |  1  |   1

How can I select uid’s with last action for example 1? In this case only last line.

UPDATE: As you can see for uid=3 at first we have action=1, then action=2 it menas that uid=3 is not “good” line for us. But for uid=1 last action is 1, so that “good” line.

UPDATE2: action=1 is enter and action=2 is exit. Finaly I need to get list of users who in building now, or at least list of last action of every user.

Answer 1

Edit: I added some explanations at the end of my answer.

DECLARE @Test TABLE(
    id INT, 
    [uid] INT, 
    [action] INT
);

INSERT  @Test(id , [uid] , [action])
SELECT  1 ,  1  ,   2
UNION ALL
SELECT  2 ,  3  ,   1
UNION ALL
SELECT  3 ,  2  ,   2
UNION ALL
SELECT  4 ,  3  ,   2
UNION ALL
SELECT  5 ,  1  ,   1;

DECLARE @last_action INT;
SET     @last_action = 1; --or = 2
SELECT  x.uid
FROM
(
SELECT  t.uid, 
        t.action,
        ROW_NUMBER() OVER(PARTITION BY t.[uid] ORDER BY t.id DESC) AS RowNum
FROM    @Test t
) x
WHERE   x.RowNum = 1
AND     x.action = @last_action 

Results:

uid         
----------- 
1          

Explanations:
The query from derived table (...) x will produce folowing results:

id uid action RowNum
-- --- ------ ------
5  1   1      1 <-- last action (1) for uid = 1
1  1   2      2
-- --- ------ ------
3  2   2      1 <-- last action (2) for uid = 2
-- --- ------ ------
4  3   2      1 <-- last action (2) for uid = 3
2  3   1      2
-- --- ------ ------