Delete rows following a duplicate - sql

Question

I have a list of user login and logout stamps. Unfortunately a LOGIN entry might not always be followed by a LOGOUT entry.
I wish to delete any row which has the same [event] and [user_id] as previous row when ordered by [event_date]
Any suggestions on how to do this?
Example table
CREATE TABLE #LOG (
[id] int IDENTITY(1,1),
[user_id] int,
[event] varchar(50),
[event_date] datetime
);
INSERT INTO #LOG ([user_id], [event], [event_date])
SELECT 1,'LOGIN',{ts '2010-12-15 15:31:59'}
UNION ALL SELECT 1,'LOGOUT',{ts '2010-12-15 15:32:55'}
UNION ALL SELECT 1,'LOGIN',{ts '2010-12-15 15:38:04'}
UNION ALL SELECT 1,'LOGOUT',{ts '2010-12-15 15:38:17'}
UNION ALL SELECT 1,'LOGOUT',{ts '2010-12-15 15:38:45'} -- Delete
UNION ALL SELECT 2,'LOGIN',{ts '2010-12-15 16:59:39'}
UNION ALL SELECT 2,'LOGOUT',{ts '2010-12-15 17:00:08'}
UNION ALL SELECT 2,'LOGOUT',{ts '2010-12-15 17:00:39'} -- Delete
UNION ALL SELECT 2,'LOGOUT',{ts '2010-12-15 17:01:16'} -- Delete
UNION ALL SELECT 2,'LOGIN',{ts '2010-12-15 17:01:38'}
UNION ALL SELECT 2,'LOGIN',{ts '2010-12-15 17:02:26'} -- Delete
UNION ALL SELECT 2,'LOGOUT',{ts '2010-12-15 17:02:39'}

Answer 1

;WITH T1 AS
(
SELECT * ,
ROW_NUMBER() OVER (ORDER BY event_date)-
ROW_NUMBER() OVER (PARTITION BY [user_id], [event]
ORDER BY event_date) AS Grp
FROM #LOG
),T2 AS
(
SELECT
ROW_NUMBER() OVER (PARTITION BY [user_id], [event], Grp
ORDER BY event_date) RN
FROM T1
)
DELETE FROM T2
WHERE RN > 1

Answer 2

Using the ROW_NUMBER functionality of SQL Server would be an option
SQL Statement
;WITH q AS (
SELECT Rownumber = ROW_NUMBER() OVER (ORDER BY user_id, event_date)
, user_id
, event
, event_date
FROM #LOG
)
DELETE FROM #LOG
FROM #LOG l
INNER JOIN (
SELECT q2.*
FROM q q1
INNER JOIN q q2 ON q2.Rownumber = q1.Rownumber + 1
AND q2.user_id = q1.user_id
AND q2.event = q1.event
) q ON q.user_id = l.user_id
AND q.event_date = l.event_date
SELECT *
FROM #LOG

Answer 3

My understanding is that you want to delete entries such that the pattern is always In,Out,In,Out,etc.
This means a record is deleted if the preceding record (when order by user_id, then event_date), is of the same event.
There are two options I'd use to go about this...
DELETE
#log
WHERE
event = (
SELECT
TOP 1
event
FROM
#log AS [preceding]
WHERE
[preceding].user_id = #log.user_id
AND [preceding].event_date < #log.event_date
ORDER BY
[preceding].event_date DESC
)
Or...
WITH ordered_log AS (
SELECT
ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY event_date) AS user_event_id,
*
FROM
#log
)
DELETE
ordered_log
FROM
ordered_log
INNER JOIN
ordered_log AS [preceding]
ON [preceding].login_id = [ordered_log].login_id
AND [preceding].user_event_id = [ordered_log].user_event_id - 1
WHERE
[preceding].event = [ordered_log].event
Either way, I highly recommend an Index covering user_id then event_date.
Note: The first version does not cope with the possibility of two events having the same timestamp. The latter, however, does.

Answer 4

If you have to delete duplicate row. Then no need to set the order by clause.
Try below
Delete l from #LOG l
Inner Join
(
Select id from #LOG l
Inner Join(
Select user_id, event from #LOG
group by user_id, event
having COUNT(user_id) > 1 and COUNT(event) > 1
)T
on (l.user_id = t.user_id) and (l.event = t.event)
)T
on T.id = l.id