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simple query question

Posted on 2008-10-29
23
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Last Modified: 2013-12-19
Say i have a table as follows:

create table test (x date);

test
____
2008-10-20 15:15:15
2008-10-20 15:15:16
2008-10-20 15:15:17
2008-10-20 15:15:18
2008-10-20 15:15:35
2008-10-20 15:15:36
2008-10-20 15:15:37
2008-10-20 15:15:38
2008-10-20 15:16:15
2008-10-20 15:16:16
2008-10-20 15:16:17
2008-10-20 15:16:18
2008-10-20 15:16:19

so there are 3 distinct periods... what i want is a query that will return the following:

sequence, start, end
1, 2008-10-20 15:15:15, 2008-10-20 15:15:18
2, 2008-10-20 15:15:35, 2008-10-20 15:15:38
3, 2008-10-20 15:16:15, 2008-10-20 15:16:19

the break between continuous records should be a number of seconds that i an choose.

Thanks in advance.
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Question by:asiminator
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23 Comments
 
LVL 27

Expert Comment

by:sujith80
ID: 22837408
The following query divides values in groups of 30 seconds.

select min(x), max(x) 
from test
group by trunc(x,'mi') + case when to_number(to_char(x,'ss')) <= 30 then 30/60/60/24 else 60/60/60/24 end 
order by 1;

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Expert Comment

by:jamesgu
ID: 22837601
not quite sure on the part - 'the break between continuous records should be a number of seconds that i an choose.'

can you give some more details?

try this for the 3 distinct periods
select s.num, t.x, s.x from (
	select row_number() over (order by x) num, x from (
	select x from test
	where not exists (
	    select 1 from (select x - 1/60/60/24 as y from test)
	    where x = y
	)
	order by x
	)
) s,
(
	select row_number() over (order by x) num, x from (
	select x from test
	where not exists (
	    select 1 from (select x + 1/60/60/24 as y from test)
	    where x = y
	)
	order by x
	)
) t
where s.num = t.num;

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Expert Comment

by:awking00
ID: 22841647
See attached.
distinct-periods.txt
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LVL 73

Expert Comment

by:sdstuber
ID: 22842370
try these....  They should be very efficient (albeit new syntax for a lot of people) since they only need to query the data one time
SELECT DISTINCT DENSE_RANK() OVER (ORDER BY minx) seq, minx, MAX(x) OVER (PARTITION BY minx) maxx
  FROM (SELECT *
          FROM (SELECT x, TO_DATE(NULL) minx, TO_DATE(NULL) maxx, ROW_NUMBER() OVER (ORDER BY x) rn
                  FROM test)
        MODEL
            DIMENSION BY(rn)
            MEASURES(x, minx)
            RULES UPDATE AUTOMATIC ORDER
                (minx [ANY]
                    ORDER BY x =
                        CASE
                            WHEN x[CV(rn)] <= x[CV(rn) - 1] + 1 / 86400 THEN minx[CV(rn) - 1]
                            ELSE x[CV(rn)]
                        END))
 
.....
 
SELECT ROWNUM seq, minx, maxx
  FROM (  SELECT minx, MAX(x) maxx
            FROM (SELECT *
                    FROM (SELECT x,
                                 TO_DATE(NULL) minx,
                                 TO_DATE(NULL) maxx,
                                 ROW_NUMBER() OVER (ORDER BY x) rn
                            FROM test)
                  MODEL
                      DIMENSION BY(rn)
                      MEASURES(x, minx)
                      RULES UPDATE AUTOMATIC ORDER
                          (minx [ANY]
                              ORDER BY x =
                                  CASE
                                      WHEN x[CV(rn)] <= x[CV(rn) - 1] + 1 / 86400
                                      THEN
                                          minx[CV(rn) - 1]
                                      ELSE
                                          x[CV(rn)]
                                  END))
        GROUP BY minx)

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Expert Comment

by:sdstuber
ID: 22842381
if you want to adjust the threshold for neighboring data,  change the

1/86400 to N/86400 where N is whatever threshold, in seconds, you want
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Expert Comment

by:sdstuber
ID: 22842417
you can remove the to_date(null)maxx,  that's a carryover from an earlier test
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Author Comment

by:asiminator
ID: 22842694
im checking the solutiions and ill come back soon
0
 
LVL 9

Accepted Solution

by:
jamesgu earned 250 total points
ID: 22842876
a simpler one
select ceil(num / 2), min(x), max(x)
from (
	select row_number() over (order by x) num, t.x from (
		select x, lead(x) over (order by x) next, lag(x) over (order by x) previous
		from test
	) t
	where (next - x > 1/60/60/24 or next is null)
	or (x - previous > 1/60/60/24 or previous is null)
) TEMP
group by ceil(num / 2)
;

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Expert Comment

by:sdstuber
ID: 22842985
jamesgu,  very nice!

More efficient than any proposed thus far
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Expert Comment

by:sdstuber
ID: 22843123
by efficient, I don't just mean smaller code,
but actually doing less work to get the same output.

jamesgu's version performs fewer io (same as mine) and fewer sorts (less than anybody else's)
0
 

Author Closing Comment

by:asiminator
ID: 31511499
Thanks to all that answered but this was by far the best solution.  Thank you all for yor efforts
0
 
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Expert Comment

by:jamesgu
ID: 22843644
thanks sdstuber

and thanks asiminator, it was an interesting question (but not simple as in the title ;)
0
 
LVL 73

Expert Comment

by:sdstuber
ID: 22843881
oops!!!

Found a bug.  jamesgu's version doesn't handle isolated rows correctly.

for instance

given the above data in table test,  add these two rows....

 insert into test (x) values (sysdate);
 insert into test (x) values (sysdate+1);

I still like the cleverness in jamesgu's version, but I'm afraid I'll have to retract my recommendation and suggest my own again (post 22842370)

Fast is good but  correct is better.  :)
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LVL 73

Expert Comment

by:sdstuber
ID: 22844035
also,  if it's possible that X might have non-unique values then I suggest using this...


  SELECT DISTINCT DENSE_RANK() OVER (ORDER BY minx) seq, minx, MAX(x) OVER (PARTITION BY minx) maxx
    FROM (SELECT *
            FROM (SELECT x, TO_DATE(NULL) minx, ROW_NUMBER() OVER (ORDER BY x) rn FROM test)
          MODEL
              DIMENSION BY(rn)
              MEASURES(x, minx)
              RULES UPDATE AUTOMATIC ORDER
                  (minx [ANY]
                      ORDER BY rn =
                          CASE
                              WHEN x[CV(rn)] <= x[CV(rn) - 1] + 1 / 86400 THEN minx[CV(rn) - 1]
                              ELSE x[CV(rn)]
                          END))
ORDER BY 1;

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Expert Comment

by:sdstuber
ID: 22844057
asiminator

I also suggest clicking the "Request Attention" link above to have the admins reopen the question and we can all hammer out the best solution.
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Expert Comment

by:jamesgu
ID: 22844188
sdstuber, good catch,

more code has to be added to handle this, this is the partial code,
	select row_number() over (order by x) num, t.x, case when (next - x > 1/60/60/24 and x - previous > 1/60/60/24)
	                                                       or (next is null and x - previous > 1/60/60/24)
	                                                       or (next - x > 1/60/60/24 and previous is null)
	                                                       then 1
	                                                     else 0 end isolated
    from (
		select x, lead(x) over (order by x) next, lag(x) over (order by x) previous
		from test
		group by x
	) t
	where (next - x > 1/60/60/24 or next is null)
	or (x - previous > 1/60/60/24 or previous is null)

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Expert Comment

by:sdstuber
ID: 22844288
I have submitted a request for attention for this.  I don't know if the question will be reopened though, since the asker really should be the one to do so, but with a demonstrably incorrect answer accepted, hopefully this is a valid exception to the norm.
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Expert Comment

by:awking00
ID: 22844580
Then mine also still works :-)
0
 
LVL 73

Expert Comment

by:sdstuber
ID: 22844632
yes, sorry, I should have stated yours was correct too.

I was recommending mine over yours simply because it was more efficient (less io)
so it will scale better on realistic datasets.
0
 

Author Comment

by:asiminator
ID: 22844732
Hey...

For my case this query is giving the results  expected.  There is a unique key on the date and i never get one value at a time... they always come in groups.

Im hesitant to reopen the question when my need filled

Asim
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Expert Comment

by:sdstuber
ID: 22844859
then do not.

having explained the reasons why the holes in jamesgu's query can't happen, I will re-reverse my suggestions and again support jamesgu's post you already accepted.

I will add one caveat.  "they always come in groups" ,  how can you enforce that?

and even if they do come in groups.  Do they always come in groups of 1 second separation?

If not,  what happens if you get

2008-10-21 15:16:19
2008-10-21 15:16:22
2008-10-21 15:16:25

You might consider those a group but if you pick a threshold of 1 second (like in the original question) they will fail with jamesgu's query.

Or are there other contraints we aren't aware of to prevent such a thing?




0
 

Author Comment

by:asiminator
ID: 22844993
You are right... there are a lot of other constraints.  i simplified my question so as not to confuse the matter.  I hate reading long winded complex questions on here.  this is part of a huge table in a huge database which gets data from a custom measurement system. Plus the data is being scrubbed before it is loaded.  

Thanks for all your help.
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Expert Comment

by:sdstuber
ID: 22845074
glad you got your answer, and thank you for returning to explain the exceptions that make the accepted answer correct.

that will help future readers of this PAQ and hopefully the other queries posted will help others too.
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