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query and unique_key_value()

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  • query and unique_key_value()

    A table has values like this
    ID	Date
    1	1-1-15
    1	3-1-15
    1	2-1-15
    2	2-1-15
    2	3-1-15
    I need a DBF query to return the rows with the most recent date for each ID. In a report this is easy but I am drawing a blank for a query.


  • #2
    Re: query and unique_key_value()

    I'm not sure there is a filter expression that will do that. Is marking records a problem?

    You could script a little

    unmark all
    order by id and invert(date)
    loop through marking the first id in each group

    filter for marked()

    You can retrieve a list of values, for the records with the most recent dates, from the table with

    table.external_record_content_get("your_table_name","id+' '+date","id+invert(date)",".t. .and. unique_key_value()")

    I suppose one could dim a global variable, populate it with the expression, and filter for values in the variable using is_one_of().
    Last edited by Stan Mathews; 04-15-2015, 09:27 AM.
    There can be only one.


    • #3
      Re: query and unique_key_value()

      Hi Stan,

      Was hoping to find a direct query vs. looping through and marking the records.

      My original approach was a set with parent_id linked 1-1 (first record) with child table (shown above). Child is sorted by descending date.

      That works fine on development box w/ multiple instances open, but in production there is an error "adding relation child child_id - table in use in another session" when a report is run on that set. So I was hoping to directly query the child table to mimic what the set was doing.



      • #4
        Re: query and unique_key_value()

        Hey Bill,

        Any of the things that I tried either didn't work at all or worked in the expression builder but resulted in a "you have specified and invalid query...." message.
        There can be only one.


        • #5
          Re: query and unique_key_value()


          When I read your first message, for some reason I did not see the section with table.external_record_content_get(). I started with that before posting, but since it returns unique key values, there will be a value for each date, and therefore duplicate ids. I am trying another test with the set and will report back.



          • #6
            Re: query and unique_key_value()

            but since it returns unique key values, there will be a value for each date
            I ordered by id and invert(date). That becomes the key value so unique_key_values() returns one id and date combination for the record with the most recent date.
            There can be only one.


            • #7
              Re: query and unique_key_value()

              Right, and I need one value for each unique id.

              We just finished a test that seems to work, but it leaves unanswered the query question in this thread.

              The child table name had a space in it. I set up a test with no space in the name and we no longer get the "open in another session" error. This table is 15+ years old and I had just not wanted to mess with changing the name. For now I will copy to a temp table each time the report is needed, until I have time to adjust all the existing references.



              • #8
                Re: query and unique_key_value()

                I would have suggested a summary operation if I knew you were considering a temp table.

                Summarize on id (group by) and the operator on the date field would be Max.
                There can be only one.


                • #9
                  Re: query and unique_key_value()

                  The summary operation is a good thought, and I did not have that thought. :-) But I do hope to rename the source table soon and bypass the need for the temp table.

                  Thanks Stan.


                  • #10
                    Re: query and unique_key_value()

                    Bill, that error can be due to the file is in the registry - try those functions to clean and see if that fixes it.
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