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Postgresql – Postgres group rows based on data in one row

I am trying to group rows on a postgres query. I have one row of data that has a field with a line number in it. The next 3 rows are null. I want to set them to the same value as the first row. Then the 5th row repeats the process. There may be 1 or more rows with null as the line number.

Here is a sample of the data with a simple query to pull the rows I want, ordered by line_num:

item_id ordered_qty unit line_num group_line_num code
NP4484140T 11 PST^ 7 7
COL48LPT 6 PST 8 null NP4484140T
COL48EPT 4 PST 9 null NP4484140T
COL48BPT 1 PST 10 null NP4484140T
VP5578135T 1 PST^ 52 52
ONTP48CPT 1 PST 53 null VP5578135T

I want group_line_num to be 7 where it is null in the first set and 52 on the 4th null. The groups can also be created based on where the unit changes with the PST^ being the defining line of the group. I have tried using lag but since the number of null lines is variable I am not sure how to make it work.

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2

Answers


  1. 0

    Although it is possible to solve this with window functions it is not entirely straightforward and I think that the simplest solution is to self join the previous rows and identify the row with the highest value of group_line_num which can be used to replace a null value in the current row.

    create table example (
    item_id varchar not null,
    ordered_qty integer not null,
    unit varchar not null,
    line_num integer not null,
    group_line_num integer,
    code varchar not null
    );

insert into example (item_id, ordered_qty, unit, line_num, group_line_num, code)
values ('NP4484140T', 11, 'PST^', 7, 7, ''),    
       ('COL48LPT', 6, 'PST', 8, null, 'NP4484140T'),
       ('COL48EPT', 4, 'PST', 9, null, 'NP4484140T'),
       ('COL48BPT', 1, 'PST', 10, null, 'NP4484140T'),
       ('VP5578135T', 1, 'PST^', 52, 52, ''),
       ('ONTP48CPT', 1, 'PST', 53, null, 'VP5578135T');

select item_id,
       ordered_qty,
       unit,
       line_num,
       coalesce(example.group_line_num, previous.group_line_num) as group_line_num,
       code
from example
left join lateral (select previous.group_line_num
                   from example as previous
                   where previous.line_num < example.line_num
                   order by previous.group_line_num desc nulls last
                   limit 1) as previous on true
order by line_num;
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  2. 0

    The described results can be easily achieved using MAX as a window function.

    The following code establishes a table of example values:

    CREATE TABLE examples (
  item_id TEXT NOT NULL,
  ordered_qty INTEGER NOT NULL,
  unit TEXT NOT NULL,
  line_num INTEGER NOT NULL,
  group_line_num INTEGER,
  code TEXT
);

INSERT INTO
  examples (
    item_id,
    ordered_qty,
    unit,
    line_num,
    group_line_num,
    code
  )
VALUES
  ('NP4484140T', 11, 'PST^', 7, 7, ''),
  ('COL48LPT', 6, 'PST', 8, NULL, 'NP4484140T'),
  ('COL48EPT', 4, 'PST', 9, NULL, 'NP4484140T'),
  ('COL48BPT', 1, 'PST', 10, NULL, 'NP4484140T'),
  ('VP5578135T', 1, 'PST^', 52, 52, ''),
  ('ONTP48CPT', 1, 'PST', 53, NULL, 'VP5578135T');

    To determine the group_line_num using it’s most recent non-NULL value, run the following:

    SELECT
  item_id,
  ordered_qty,
  unit,
  line_num,
  MAX(group_line_num) OVER (ORDER BY line_num) AS group_line_num,
  code
FROM
  examples
ORDER BY
  line_num;

    The following query derives group_line_num from the most recent line_num associated with unit = 'PST^':

    SELECT
  item_id,
  ordered_qty,
  unit,
  line_num,
  MAX(line_num) FILTER (WHERE unit = 'PST^') OVER (ORDER BY line_num) AS group_line_num,
  code
FROM
  examples
ORDER BY
  line_num;

    The two queries return identical results:

    item_id ordered_qty unit line_num group_line_num code
    NP4484140T 11 PST^ 7 7 null
    COL48LPT 6 PST 8 7 NP4484140T
    COL48EPT 4 PST 9 7 NP4484140T
    COL48BPT 1 PST 10 7 NP4484140T
    VP5578135T 1 PST^ 52 52
    ONTP48CPT 1 PST 53 52 VP5578135T
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