Make two columns unique both ways

I'm creating a database which stores football matches.

Each round, any given football club may only play once (home or away).

For example, after the insert of

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

inserts such as

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

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

must be forbidden since Barcelona has already played one match against Real in Round 1. They shouldn't be able to play against Bayern in the same round (doesn't matter if they're playing home or away).

How can I ensure such inserts don't happen?

Do I use a trigger? Some sort of unique index combination? I've tried both but got lost and didn't actually get the outcome expected.

Thanks

With 2 unique constraints you can achieve the desired restriction :

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

Check the fiddle here

StackExchange Database Administrators Q#320513, answer score: 4