Delete rows following a duplicate

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