Re: select from grouped data

Greetings,

* Roger Mason (rmason@mun.ca) wrote:
I have written this function to process some multiline text data into a
table with both text & numeric columns:

CREATE OR REPLACE FUNCTION get_final_energy ()
  RETURNS TABLE (
    jid text,
    "timestamp" text,
    scf integer,
    energy double precision
  )
  AS $$
  WITH a AS (
    SELECT
      jid,
      regexp_split_to_table(totenergy_out, '\n') AS teo
    FROM
      results
),
b AS (
  SELECT
    results.jid,
    results. "timestamp",
    cast(
      CASE WHEN split_part(a.teo, ' ', 2) = '' THEN
        '0'
      ELSE
        split_part(a.teo, ' ', 2)
      END AS integer) AS scf,
    cast(
      CASE WHEN split_part(a.teo, ' ', 3) = '' THEN
        '0.0'
      ELSE
        split_part(a.teo, ' ', 3)
      END AS double precision) AS energy
  FROM
    results,
    a
  WHERE
    results.jid = a.jid
  GROUP BY
    results.jid,
    results. "timestamp",
    a.teo
  --HAVING
    -- scf = max(scf)
    ORDER BY
         timestamp ASC,
         scf DESC
)
SELECT
  *
FROM
  b;

$$
LANGUAGE sql;

The output looks like:

   jid     |    timestamp    | scf |     energy     
------------+-----------------+-----+----------------
 1250_1     | 20210805-114634 |  18 | -1316.43700819
 1250_1     | 20210805-114634 |  17 | -1316.43700825
 1250_1     | 20210805-114634 |  16 |  -1316.4370097
 1250_1     | 20210805-114634 |  15 | -1316.43700991
 1250_1     | 20210805-114634 |  14 | -1316.43699775
 1250_1     | 20210805-114634 |  13 | -1316.43699117
 1250_1     | 20210805-114634 |  12 | -1316.43750771
 1250_1     | 20210805-114634 |  11 | -1316.43805358
 1250_1     | 20210805-114634 |  10 | -1316.43857192
 1250_1     | 20210805-114634 |   9 | -1316.43070942
 1251_1     | 20210806-062539 |  18 | -1316.43700819
 1251_1     | 20210806-062539 |  17 | -1316.43700826
....

What I want is to get (for each group) the energy corresponding to the
maximum value of scf.

* David G. Johnston (david.g.johnston@gmail.com) wrote:


 The output looks like:

    jid     |    timestamp    | scf |     energy
 ------------+-----------------+-----+----------------
  1250_1     | 20210805-114634 |  18 | -1316.43700819
  1250_1     | 20210805-114634 |  17 | -1316.43700825
  1250_1     | 20210805-114634 |  16 |  -1316.4370097
  1250_1     | 20210805-114634 |  15 | -1316.43700991
  1250_1     | 20210805-114634 |  14 | -1316.43699775
  1250_1     | 20210805-114634 |  13 | -1316.43699117
  1250_1     | 20210805-114634 |  12 | -1316.43750771
  1250_1     | 20210805-114634 |  11 | -1316.43805358
  1250_1     | 20210805-114634 |  10 | -1316.43857192
  1250_1     | 20210805-114634 |   9 | -1316.43070942
  1251_1     | 20210806-062539 |  18 | -1316.43700819
  1251_1     | 20210806-062539 |  17 | -1316.43700826
 ....

 What I want is to get (for each group) the energy corresponding to the
 maximum value of scf.


SELECT DISTINCT ON (jid) jid, timestamp, scf, energy [...] ORDER BY jid,
scf DESC

While this works, it's generally better to use a LATERAL join as that's
part of the SQL standard while DISTINCT ON isn't.  Using a LATERAL join
also would allow you to have multiple rows (top-N) if you wanted.  You'd
do that using:

WITH jids AS (SELECT jid FROM results GROUP BY jid)
SELECT jids.jid, t.ts, t.scf, t.energy
FROM jids CROSS JOIN LATERAL
 (SELECT ts, scf, energy FROM results
  WHERE results.jid = jids.jid
  ORDER BY scf DESC
  LIMIT 1) AS t
;

A couple of notes on this: don't name a column "timestamp" and when it's
a timestamp, use the 'timestamptz' data type, not text.  Your jids sure
look like they should just be numbers instead of text too.  If you
already have a distinct set of jids somewhere (like in another table),
you could use that as the source table instead of the CTE that I'm using
above.

Thanks,

Stephen