Question

I am stuck figuring out a working SQL Query for the fallowing:
I need to generate a Fire Register report (how many people are still inside the building) based on an Access database that records login/logout events along with some metadata.
The Access DB looks like this:

+----+---------------------+---------+---------+------+
| id | date                | action  | success | user |
+----+---------------------+---------+---------+------+
|  1 | 2009-04-28 02:00:00 |   login |       1 | Nick |
|  2 | 2009-04-28 03:00:00 |  logout |       1 | Nick |
|  3 | 2009-04-28 04:00:00 |   login |       1 | Nick |
|  4 | 2009-04-28 04:00:00 |  logout |       1 | Nick |
|  5 | 2009-04-28 04:00:00 |   login |       1 | Nick |
|  6 | 2009-04-28 07:00:00 |   login |       1 | John |
|  7 | 2009-04-28 07:30:00 |   login |       1 | Sue  |
|  8 | 2009-04-28 08:00:00 |  logout |       1 | John |
+----+---------------------+---------+---------+------+

During the day there can be multiple login/logout actions.
When administrator runs the report, it’s only limited for current day and needs to list all users where the last known action for this user is login and success=1, meaning that this person is currently in the building.

On the data above, Nick and Sue must be pointed out as still being inside the building.

Answer 1

Another approach to the problem:

SELECT
     T1.user
FROM
     Some_Table T1
LEFT OUTER JOIN Some_Table T2 ON
     T2.user = T1.user AND
     T2.success = 1 AND
     T2.date > T1.date
WHERE
     T1.success = 1 AND
     T1.action = 'login' AND
     T2.id IS NULL

This assumes that you only care about successful actions. Also, if a user has two actions on the same EXACT date AND time then it might not act as expected.