Postgres 15

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select version();

version

PostgreSQL 15.0 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10), 64-bit

SELECT 1

create table Match (
  Round int,
  Home varchar(25),
  Away varchar(25) );

CREATE TABLE

create unique index on match (round, least(home, away));

create unique index on match (round, greatest(home, away));

CREATE INDEX

CREATE INDEX

INSERT INTO Match (Round, Home, Away) VALUES (1, 'Barcelona', 'Real');

INSERT 0 1

INSERT INTO Match (Round, Home, Away) VALUES (1, 'Bayern', 'Barcelona');

ERROR:  duplicate key value violates unique constraint "match_round_least_idx"
DETAIL:  Key (round, LEAST(home, away))=(1, Barcelona) already exists.

INSERT INTO Match (Round, Home, Away) VALUES (1, 'Real', 'Bayern');

ERROR:  duplicate key value violates unique constraint "match_round_greatest_idx"
DETAIL:  Key (round, GREATEST(home, away))=(1, Real) already exists.

INSERT INTO Match (Round, Home, Away) VALUES (1, 'Barcelona', 'Bayern');

ERROR:  duplicate key value violates unique constraint "match_round_least_idx"
DETAIL:  Key (round, LEAST(home, away))=(1, Barcelona) already exists.

select * from Match;

round
home
away

1
Barcelona
Real

SELECT 1