Problem in query writting , Need a help on priority

Question

Hi there,
Please check the following scenario
there are 2 tables
event - eventId , userId and eventDate
impression - impressionId,userId
After joining -
userId | impressionId | eventId | eventDate
1 | 1 | null | null
1 | 2 | null | null
1 | 3 | 1 | 2007/08/10
1 | 4 | null | null
1 | 5 | null | null
1 | 6 | null | null
1 | 7 | 2 | 2007/08/10
I want the following output to be generated.
userId | impressionPosition | eventCnt
1 | 3 | 1
1 | 4 | 1 (2nd event occured AFTER 4 impressions)
Please give some query clue or sample query.
Thanks in Adv.
-Amol Kashid

Answer 1

you just need to add a partition clause to the analytic function:
with t as (select 1 as userid, 1 as impressionid, null as eventid, null as eventdate from dual
union all select 1, 2,null, null from dual
union all select 1, 3,1, sysdate-5 from dual
union all select 1, 4,null, null from dual
union all select 1, 5,null, null from dual
union all select 2, 6,2, sysdate from dual
union all select 1, 7,2, sysdate from dual
union all select 2, 8,2, sysdate from dual)
select userid, count(*) over(partition by userid order by impressionid) event_sequence,
                                      'Event after '||
                                      to_char(nvl(impressionid - lag(impressionid) over (partition by userid order by impressionid),0))
                                      ||' impression(s)' as description
  from t
  where eventid is not null
    USERID EVENT_SEQUENCE DESCRIPTION
         1              1 Event after 0 impression(s)
         1              2 Event after 4 impression(s)
         2              1 Event after 0 impression(s)
         2              2 Event after 2 impression(s)
4 rows selected.edit: actually, that screwed up the reporting of the first event.... hmmm....