Question

SQL Query

Asked by: davelane

Couldn't find a forum just for SQL so I'll ask here as the DB I am using is Oracle.

OK. I have a fairly large table with say 5000 entries in the following form:

GISID      DATATYPE     TIMESTAMP       ID         VALUE  
123         ABC              12:00                1           12345
123         ABC              12:05                2           23456
123         ABC              12:10                3           34567
123         XYZ              12:00                4           45678
123         XYZ              12:05                5           56789
999         ABC              12:00                6           67890
999         XYZ              12:05                7           78901
etc.

I want to do a single select that returns me the latest (i.e. with the newest timestamp) for each unique pair of GISID and DATATYPE. I.e. that would return

123         ABC              12:10                3           34567
123         XYZ              12:05                5           56789
999         ABC              12:00                6           67890
999         XYZ              12:05                7           78901

If I implement that programatically it leads to as many SQL queries as there are unique pairings of GISID and DATATYPE, which will be large. There are typically at most 3 values for a pairing at any given time in the table.

Something like
  SELECT GISID, DATATYPE, Max(TIMESTAMP)
   FROM TABLEX
   GROUP BY GISID, DATATYPE

Will give me the 3 fields GISID, DATATYPE and TIMESTAMP, but I need all the data for a row.

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Asked On
2004-08-04 at 02:47:40ID21081697
Topic

Oracle Database

Participating Experts
2
Points
250
Comments
9

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Answers

 

by: orageniusPosted on 2004-08-04 at 03:34:20ID: 11713851

select * from tablex where ( GISID, DATATYPE, TIMESTAMP ) in
  ( select GISID, DATATYPE, max ( TIMESTAMP ) from tablex group by GISID, DATATYPE )
/

 

by: fransenpPosted on 2004-08-04 at 03:42:17ID: 11713889

Question is ... is the time stamp unique for each GISID and DATATYPE.
I doubt it ... so there most be something else destiguish which wone is the latest entry. Maybe the ID is populated from a sequence and the hightst id for a GISID, DATATYPE and TIME STAMP is the last entry ...

Then a solution would be

select tableX.GISID, tableX.DATATYPE, tableX.TIMESTAMP, tableX.ID, tableX.VALUE
FROM tableX
, (select GISID, DATATYPE, MAX(TIMESTAMP) TS, max(ID) I
   FROM tableX
   group by GISID, DATATYPE
) tabley
where tableX.GISID = tableX.GISID
,         tableX.DATATYPE = tableY.DATATYPE
,         tableX.TIMESTAMP = tableY.TS
,         tableX.ID = tableY.I

If the timestamp is unique then simplify the quiry a little
select tableX.GISID, tableX.DATATYPE, tableX.TIMESTAMP, tableX.ID, tableX.VALUE
FROM tableX
, (select GISID, DATATYPE, MAX(TIMESTAMP) TS
   FROM tableX
   group by GISID, DATATYPE
) tabley
where tableX.GISID = tableX.GISID
,         tableX.DATATYPE = tableY.DATATYPE
,         tableX.TIMESTAMP = tableY.TS

 

by: davelanePosted on 2004-08-04 at 03:45:43ID: 11713906

Great, thanks. (Should have thought of that myself :) )

The problem with the solution is the following. The selected data will be transfered into another table where there is a unique key over the GISID and DATATYPE fields. If the data contained for some reason the following:

123         ABC              12:00                1           12345
123         ABC              12:00                2           23456
123         ABC              12:00                3           34567

Then it would select all of the 3 entries. Is there any way to guarantee only one is returned. Doesn't matter which, just so that it doesn't hit the contraint.

 

by: davelanePosted on 2004-08-04 at 03:47:46ID: 11713923

The above comment relates to the first proposed solution.

....

 

by: fransenpPosted on 2004-08-04 at 03:53:30ID: 11713964

my comment and oragenius comment are basically the same ... mine is just a longer way to write the same thing (to me clearer)

So my first version should do it ... (if the ID is unique and always increasing)

To do it with oragenius query ... just add the ID

 select * from tablex where ( GISID, DATATYPE, TIMESTAMP, ID ) in
  ( select GISID, DATATYPE, max ( TIMESTAMP) , max(ID)  from tablex group by GISID, DATATYPE )

 

by: orageniusPosted on 2004-08-04 at 03:55:39ID: 11713971

if the timestamp column is of date / timestamp datatype then there are very rare chance of that to happen still you can alter the query so that it wont violate the constraint

select * from
  (select rownum rn, * from tablex where ( GISID, DATATYPE, TIMESTAMP ) in
    ( select GISID, DATATYPE, max ( TIMESTAMP ) from tablex group by GISID, DATATYPE )
  group by GISID, DATATYPE )
where rn = 1
/

but try to avoid it if possible.

 

by: davelanePosted on 2004-08-04 at 04:00:41ID: 11713998

OK, cheers guys, that should be enough to work on!

 

by: orageniusPosted on 2004-08-04 at 04:16:41ID: 11714087

ok even the question is closed; wat if id is not unique???

123         ABC              12:00                1           12345
123         ABC              12:00                2           23456
123         ABC              12:00                2           34567

select * from tablex where ( GISID, DATATYPE, TIMESTAMP, ID ) in
  ( select GISID, DATATYPE, max ( TIMESTAMP) , max(ID)  from tablex group by GISID, DATATYPE )

will return 2 records for the same GISID and DATATYPE.

Also issues with join [ initial solution given by fransenp ]... ll create a scenario and post the result.

 

by: fransenpPosted on 2004-08-04 at 05:14:22ID: 11714477

If no ID or something else adds uniqueness to the row it cant (in my knowledge) be done in a single query but instead through two looping cursors where the first one is the group query and the second one takes the first row from tableX where the conditions for the group query is filled

or add a artificially unique column (that can be populated from a sequence) to tableX and then use it as ID was used before

still there is nothing saying which row you will get between
123         ABC              12:00                2           23456
123         ABC              12:00                2           34567

20120131-EE-VQP-002

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