Postgres 16

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CREATE TABLE t(
   city1 VARCHAR(32) NOT NULL
  ,city2 VARCHAR(11)
  ,dist  INTEGER 
);

CREATE TABLE

INSERT INTO t(city1,city2,dist) VALUES ('New York','Berlin',7900);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Berlin','New York',7900);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Oregon','Ohio',5700);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Montreal','Rio',5700);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Ohio','Oregon',5700);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Rio','Montreal',5700);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Moscow','Tokyo',4200);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Tokyo','Moscow',4200);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Conce','Santiago',500);

INSERT 0 1

INSERT INTO t(city1,city2,dist) VALUES ('Talca','Linares',80);

INSERT 0 1

select a.*
from t a
left join t b on b.city1 = a.city2 and b.city2 = a.city1
where b.city1 is null or b.city1 > b.city2

city1
city2
dist

Talca
Linares
80

Berlin
New York
7900

Ohio
Oregon
5700

Montreal
Rio
5700

Conce
Santiago
500

Moscow
Tokyo
4200

SELECT 6

select * from t;

city1
city2
dist

New York
Berlin
7900

Berlin
New York
7900

Oregon
Ohio
5700

Montreal
Rio
5700

Ohio
Oregon
5700

Rio
Montreal
5700

Moscow
Tokyo
4200

Tokyo
Moscow
4200

Conce
Santiago
500

Talca
Linares
80

SELECT 10

city1	city2	dist
Talca	Linares	80
Berlin	New York	7900
Ohio	Oregon	5700
Montreal	Rio	5700
Conce	Santiago	500
Moscow	Tokyo	4200