Help: how to compute duration between 2 dates, when these dates on 2 rows?

Question

Hello,
We are on 11g.
We have a table with times saved in one column. Then each row as a différent date value.
There is no pb to order the query on this date column (obviously)
Now, we need to compute the period of time between 2 successive rows.
I have firstly thought about using a cursor, loop on the cursor,compute and update the "period" column.
this is the basic option
Now I would like to know if, using a single query, I could directly get the computation I want ?
here is my example:
WITH t AS (
SELECT 'aaa' col1, to_date( '20100201 09:23:50', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
UNION
SELECT 'aaa' col1, to_date( '20100201 13:14:33', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
UNION
SELECT 'aaa' col1, to_date( '20100207 13:14:33', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
UNION
SELECT 'bbb' col1, to_date( '20100202 09:21:10', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
UNION
SELECT 'bbb' col1, to_date( '20100203 08:11:06', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
UNION
SELECT 'bbb' col1, to_date( '20100203 15:13:55', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
UNION
SELECT 'bbb' col1, to_date( '20100210 10:14:27', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
SELECT col1, to_char( date1,'YYYYMMDD HH24:MI:SS') date1  FROM t
ORDER BY col1, date1;ideally, this is what I'd like to get (if my computation is correct)
COL DATE1               DURATION
aaa 20100201 09:23:50          0
aaa 20100201 13:14:33      13843
aaa 20100207 13:14:33     518400
bbb 20100202 09:21:10          0
bbb 20100203 10:11:06      89396
bbb 20100203 15:13:55      18169
bbb 20100210 17:14:27     612032I hope I'm clear !!!
Thanks a lot, in advance, for your help
Olivier

Answer 1

Sure; it's easy with the use of the lag analytic function:
WITH t AS (SELECT 'aaa' col1, to_date( '20100201 09:23:50', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
           UNION ALL
           SELECT 'aaa' col1, to_date( '20100201 13:14:33', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
           UNION ALL
           SELECT 'aaa' col1, to_date( '20100207 13:14:33', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
           UNION ALL
           SELECT 'bbb' col1, to_date( '20100202 09:21:10', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
           UNION ALL
           SELECT 'bbb' col1, to_date( '20100203 08:11:06', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
           UNION ALL
           SELECT 'bbb' col1, to_date( '20100203 15:13:55', 'YYYYMMDD HH24:MI:SS') date1 FROM dual
           UNION ALL
           SELECT 'bbb' col1, to_date( '20100210 10:14:27', 'YYYYMMDD HH24:MI:SS') date1 FROM dual),
t_diff as (SELECT col1,
                  to_char( date1,'YYYYMMDD HH24:MI:SS') date1,
                  (date1 - lag(date1, 1, date1) over (partition by col1 order by date1))*24*60*60 date_diff
           FROM   t)
select col1,
       date1,
       sum(date_diff) over (partition by col1 order by date1) duration
from   t_diff
ORDER BY col1, date1;Edited by: Boneist on 15-Feb-2010 16:39
(I can't read; was originally giving the cumulative total of the duration. D'oh!)
ETA2: Take note of the additional parameters I've used in the lag() - the third parameterd handles what the value of the field should be if there is no previous row, so there's no need to use nvl elsewhere in the query to handle that situation.
Edited by: Boneist on 15-Feb-2010 16:42