GROUP BY with MAX and UNION, or JOIN?-CodePudding

How to obtain from this table date_departure and date_arrival for each travel according visiting_order

select * from step;
 id_step | id_travel | id_port | visiting_order | date_arrival | date_departure 
--------- ----------- --------- ---------------- -------------- ----------------
       1 |         1 |       1 |              0 |              | 2021-01-12
       2 |         1 |       2 |              1 | 2021-05-20   | 2021-05-22
       3 |         1 |       3 |              2 | 2021-07-27   | 
       4 |         2 |       4 |              0 |              | 2021-02-13
       5 |         2 |       5 |              1 | 2021-02-27   | 
       6 |         3 |       7 |              0 |              | 2022-01-12
       7 |         3 |       6 |              1 | 2022-05-27   |

like this :

  id_travel | date_departure | date_arrival
------------ ---------------- --------------
          1 |   2021-01-12   |  2021-07-27              
          2 |   2021-02-13   |  2021-02-27              
          3 |   2022-01-12   |  2022-05-27

My first intention was to take both columns and UNION them

   (SELECT id_travel, date_departure FROM step WHERE visiting_order = 0 
    GROUP BY id_travel, date_departure)

    UNION

   (SELECT A.id AS id_travel, A.arr_date AS date_arrival FROM 
      (SELECT id_travel, MAX(visiting_order), date_arrival
       FROM step GROUP BY id_travel
      ) AS A(id, ord, arr_date)
   );

and first select is ok

 id_travel | date_departure 
----------- ----------------
         1 | 2021-01-12
         2 | 2021-02-13
         3 | 2022-01-12

but second one return an error

 ERROR:  column "step.date_arrival" must appear in the GROUP BY clause or be used in an aggregate function

CodePudding user response：

Seems like this can just be:

SELECT id_travel
     , min(date_departure) AS date_departure
     , max(date_arrival) AS date_arrival
FROM   step
GROUP  BY 1
ORDER  BY 1;

Certainly works with your sample data.

CodePudding user response：

SELECT DISTINCT step.id_travel, sub_query1.date_departure, sub_query.date_arrival
FROM  step
INNER JOIN
    (SELECT step.id_travel, MAX(step.date_arrival) as date_arrival
     FROM step
     GROUP BY step.id_travel
    )
    AS sub_query ON (sub_query.id_travel = step.id_travel)
INNER JOIN 
    (SELECT id_travel, date_departure FROM step WHERE visiting_order = 0 GROUP BY id_travel, date_departure)
    AS sub_query1 ON (sub_query1.id_travel = step.id_travel)
ORDER BY id_travel;