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  • SQL Finds a Delta

    December 7, 2011 Hey, Ted

    Is there a way using SQL to calculate the amount a numeric column changes from one row to the next?

    –Ken

    Yes. Here’s how it’s done.

    First, you need a table to use for an example.

    create table mydata
     ( Name char(8), Qty dec(3,0))
    
    insert into mydata values
      ('Bob',   25),
      ('Bill',  85),
      ('Sue',   17),
      ('Bob',   28),
      ('Bob',   32),
      ('Bill', 105),
      ('Sue',   45)
    

    To pull this off, the data must be stored in the order in which it is to be retrieved. If necessary, create a temporary table to hold the sorted data.

    declare global temporary table SortedData as
      (select name, qty
         from mydata
        order by name, qty)
    with data
    

    Join each row to the one before it.

    with NumberedData as
       (select rrn(x) as seq, x.name, x.qty
          from session/SortedData as x)
    select after.name,
           coalesce(before.qty,0) as before,
           after.qty as after,
           after.qty - coalesce(before.qty,0) as change
      from NumberedData as after
      left join NumberedData as before
        on after.name = before.name
       and after.seq = before.seq + 1
    

    NumberedData is a common table expression. It creates a temporary table in memory, with three columns: relative record number, name, and quantity. You have to have relative record number in order to join one row of NumberedData to another row of NumberedData. If you could see NumberedData, it would look like this:

    SEQ   NAME      QTY
      1   Bill       85
      2   Bill      105
      3   Bob        25
      4   Bob        28
      5   Bob        32
      6   Sue        17
      7   Sue        45
    

    In this example, I’m tracking the change for each person. I want to see the change between records 1 and 2, but not between 2 and 3. For that reason, the first part of the join compares the name columns of two rows to be sure they match. The second part of the join checks to see that the “before” row is numbered one less than the “after” row.

    Voilà!

    NAME    BEFORE   AFTER  CHANGE
    Bill         0     85       85
    Bill        85    105       20
    Bob          0     25       25
    Bob         25     28        3
    Bob         28     32        4
    Sue          0     17       17
    Sue         17     45       28
    

    –Ted



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Volume 11, Number 37 -- December 7, 2011
THIS ISSUE SPONSORED BY:

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