Postgres 13

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create table t (
    id int,
    id2 int,
    check (id <> id2),
    exclude using gist ( (array[id, id2]) with &&)
)

CREATE TABLE

insert into t (id, id2) values (1, 2)

INSERT 0 1

insert into t (id, id2) values (3, 4)

INSERT 0 1

insert into t (id, id2) values (4, 3)

ERROR:  conflicting key value violates exclusion constraint "t_array_excl"
DETAIL:  Key ((ARRAY[id, id2]))=({4,3}) conflicts with existing key ((ARRAY[id, id2]))=({3,4}).

insert into t (id, id2) values (4, 4)

ERROR:  new row for relation "t" violates check constraint "t_check"
DETAIL:  Failing row contains (4, 4).

insert into t (id, id2) values (5, 5)

ERROR:  new row for relation "t" violates check constraint "t_check"
DETAIL:  Failing row contains (5, 5).

insert into t (id, id2) values (6, 7), (7, 8)

ERROR:  conflicting key value violates exclusion constraint "t_array_excl"
DETAIL:  Key ((ARRAY[id, id2]))=({7,8}) conflicts with existing key ((ARRAY[id, id2]))=({6,7}).

select *
from t

id
id2

1
2

3
4

SELECT 2