Re: Get the middle part of a result set

On Dec 18, 3:17 am, Jens Riedel <Jens..._at_gmx.de> wrote:
> Hi,
>
> I have to following problem:
>
> I get X rows from a statement, these are sorted by a certain column,
> let's say an numerical value.
> Now I want to calculate the average of this numerical value, but the 10%
> with the lowest and the 10% with the highest value shall not be included
> in this calculation.
> So for example, if I get 20 rows, I need the average of the value in
> rows 3 to 18.
>
> Currently I solved this with a very complicated statement, but I don't
> know the built-in Oracle mathematical functions so I hope that there
> could be a way to do this with a better performance.
>
> Thanks for any hints,
>
> Jens

Let's set up a short experiment:
CREATE TABLE T1 (C1 NUMBER(4)); INSERT INTO T1
SELECT
  ROWNUM
FROM
  DUAL
CONNECT BY
  LEVEL<=20;

We now have a table with 20 rows with numbers between 1 and 20.

Assuming that you are running a version of Oracle that supports analytical functions, the following returns the twenty rows with the relative ranking of each row, if the rows are sorted by C1 in descending order:
SELECT
  C1,
  DENSE_RANK() OVER (ORDER BY C1 DESC) DR,   COUNT(C1) OVER (PARTITION BY 1) R
FROM
  T1;

        C1 DR R
---------- ---------- ----------

        20          1         20
        19          2         20
        18          3         20
        17          4         20
        16          5         20
        15          6         20
        14          7         20
        13          8         20
        12          9         20
        11         10         20
        10         11         20
         9         12         20
         8         13         20
         7         14         20
         6         15         20
         5         16         20
         4         17         20
         3         18         20
         2         19         20
         1         20         20

A slight modification of the above, dividing the value of DENSE_RANK by the value of COUNT, and also including a PERCENT_RANK for comparison:
SELECT
  C1,
  DENSE_RANK() OVER (ORDER BY C1 DESC) DR,   (DENSE_RANK() OVER (ORDER BY C1 DESC))/(COUNT(C1) OVER (PARTITION BY 1)) DRP,
  PERCENT_RANK() OVER (ORDER BY C1 DESC) PR FROM
  T1;

        C1 DR DRP PR ---------- ---------- ---------- ----------

        20          1        .05          0
        19          2         .1 .052631579
        18          3        .15 .105263158
        17          4         .2 .157894737
        16          5        .25 .210526316
        15          6         .3 .263157895
        14          7        .35 .315789474
        13          8         .4 .368421053
        12          9        .45 .421052632
        11         10         .5 .473684211
        10         11        .55 .526315789
         9         12         .6 .578947368
         8         13        .65 .631578947
         7         14         .7 .684210526
         6         15        .75 .736842105
         5         16         .8 .789473684
         4         17        .85 .842105263
         3         18         .9 .894736842
         2         19        .95 .947368421
         1         20          1          1

The final cleanup is performed when the above is slid into an inline view, by using a WHERE clause:
SELECT
  SUM(T.C1) S
FROM
  (SELECT
    C1,
    DENSE_RANK() OVER (ORDER BY C1 DESC) DR,     (DENSE_RANK() OVER (ORDER BY C1 DESC))/(COUNT(C1) OVER (PARTITION BY 1)) DRP,
    PERCENT_RANK() OVER (ORDER BY C1 DESC) PR   FROM
    T1) T
WHERE
  T.DRP>0.1
  AND T.DRP<=0.9;

         S

       168

A version that uses the PERCENT_RANK value: SELECT
  SUM(T.C1) S
FROM
  (SELECT
    C1,
    DENSE_RANK() OVER (ORDER BY C1 DESC) DR,     (DENSE_RANK() OVER (ORDER BY C1 DESC))/(COUNT(C1) OVER (PARTITION BY 1)) DRP,
    PERCENT_RANK() OVER (ORDER BY C1 DESC) PR   FROM
    T1) T
WHERE
  T.PR BETWEEN 0.1 AND 0.9;          S

       168

You will obviously need to test the above approach with your data.

Charles Hooper
IT Manager/Oracle DBA
K&M Machine-Fabricating, Inc. Received on Tue Dec 18 2007 - 06:08:44 CST