Postgres 16

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create table zipnames (id int, zipcode varchar(100), name_fi varchar(100), name_sv varchar(100));

CREATE TABLE


insert into zipnames values 
(1,'00000',NULL,NULL),
(2,'00100','HELSINKI','HELSINGFORS'),
(3,'00100','HELSINKI',NULL),
(4,'00100',NULL,'HELSINGFORS'),
(5,'00100',NULL,NULL),
(6,'50100','MIKKELI',NULL),
(7,'50100',NULL,NULL),
(8,'64250',NULL,'PJELAX');

INSERT 0 8


SELECT
  DISTINCT ON (zipcode) 
  id, zipcode,
  name_fi,
  name_sv
FROM
  zipnames
ORDER BY
  zipcode,
  name_fi,
  name_sv;


id
zipcode
name_fi
name_sv


1
00000
null
null

2
00100
HELSINKI
HELSINGFORS

6
50100
MIKKELI
null

8
64250
null
PJELAX



SELECT 4


SELECT
  DISTINCT ON (zipcode) 
  id, zipcode,
  name_fi,
  name_sv
FROM
  zipnames
ORDER BY
  zipcode,
  name_fi NULLS LAST,
  name_sv NULLS LAST;


id
zipcode
name_fi
name_sv


1
00000
null
null

2
00100
HELSINKI
HELSINGFORS

6
50100
MIKKELI
null

8
64250
null
PJELAX



SELECT 4

id	zipcode	name_fi	name_sv
1	00000	null	null
2	00100	HELSINKI	HELSINGFORS
6	50100	MIKKELI	null
8	64250	null	PJELAX