how to get biggest result from a sql result in postgresql

Question

I am using postgresql 8.3 and I have a simple sql query:

SELECT a.id,a.bpm_process_instance_id,a.actor_id 
  FROM bpm_task_instance a
 WHERE a.bpm_process_instance_id IN 
(
   SELECT bpm_process_instance_id 
         FROM incident_info 
        WHERE status = 12
          AND registrant = 23
)

so, I got a result set like this:

id    instance_id  actor_id
150     53            24
147     53            26
148     53            25
161     57            26
160     57            26
158     57            24
165     58            23
166     58            24
167     58            24

now, I want to get the max id by instance_id, and the result is like blew

id    instance_id  actor_id
150     53            24
161     57            26
167     58            23

how could I get the result ? I use the following sql, but get an error.

ERROR: relation "x" does not exist

SELECT * 
  FROM (SELECT a.id,a.bpm_process_instance_id,a.actor_id 
          FROM bpm_task_instance a
         WHERE a.bpm_process_instance_id IN
            (
               SELECT bpm_process_instance_id 
                     FROM incident_info
                        WHERE status = 12
                      AND registrant = 23
            ) 
     ) AS x
 WHERE x.id = (
       SELECT max(id)
             FROM x 
            WHERE bpm_process_instance_id = x.bpm_process_instance_id
          )

anyone who can help me , thanks a lot!

Answer 1

DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;

CREATE TABLE the_table
        ( id INTEGER NOT NULL
        , instance_id INTEGER NOT NULL
        , actor_id INTEGER NOT NULL
        );
INSERT INTO the_table(id, instance_id, actor_id) VALUES
(150,53,24) ,(147,53,26) ,(148,53,25)
,(161,57,26) ,(160,57,26) ,(158,57,24)
,(165,58,23) ,(166,58,24) ,(167,58,24)
        ;

SELECT id, instance_id, actor_id
FROM the_table dt
WHERE NOT EXISTS (
        SELECT *
        FROM the_table nx
        WHERE nx.instance_id = dt.instance_id
        AND nx.id > dt.id
        );

Result (note: the last row differs!):

DROP SCHEMA
CREATE SCHEMA
SET
CREATE TABLE
INSERT 0 9
 id  | instance_id | actor_id 
-----+-------------+----------
 150 |          53 |       24
 161 |          57 |       26
 167 |          58 |       24
(3 rows)

UPDATE: this is the query including the other subquery and the missing table, and the original (ugly) column names, all packed into a CTE:

WITH zcte AS (
        SELECT ti.id AS id
                , ti.bpm_process_instance_id AS instance_id
                , ti.actor_id AS actor_id
        FROM bpm_task_instance ti
        WHERE EXISTS ( SELECT * FROM incident_info ii
                WHERE ii.bpm_process_instance_id = ti.bpm_process_instance_id
                AND ii.status = 12
                AND ii.registrant = 23
                )
        )
SELECT id, instance_id, actor_id
FROM zcte dt
WHERE NOT EXISTS (
        SELECT *
        FROM zcte nx
        WHERE nx.instance_id = dt.instance_id
        AND nx.id > dt.id
        );

UPDATE addendum:

Oops, the bad news is that 8.3 did not have CTE's yet. (think about upgrading). The good news is: as a workaround you could make zcte () as a (temporary) VIEW, and refer to that instead.