Postgres 12

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CREATE TABLE thing (
    id INT PRIMARY KEY
);

CREATE TABLE


INSERT INTO thing
    (id)
    VALUES (1);
INSERT INTO thing
    (id)
    VALUES (2);

INSERT 0 1



INSERT 0 1


CREATE TABLE relationship (
    thing_one INT REFERENCES thing(id),
    thing_two INT REFERENCES thing(id),
    value INT NOT NULL,
    PRIMARY KEY (thing_one, thing_two),
    CHECK (thing_one != thing_two)
);

CREATE TABLE


CREATE UNIQUE INDEX unique_pair_ix
    ON relationship (
        least(thing_one, thing_two),
        greatest(thing_one, thing_two)
    );

CREATE INDEX


INSERT INTO relationship
            (thing_one,
             thing_two,
             value)
            VALUES (1, 1, 100);

ERROR:  new row for relation "relationship" violates check constraint "relationship_check"
DETAIL:  Failing row contains (1, 1, 100).


INSERT INTO relationship
            (thing_one,
             thing_two,
             value)
            VALUES (1, 2, 97);

INSERT 0 1


INSERT INTO relationship
            (thing_one,
             thing_two,
             value)
            VALUES (2, 1, 97);

ERROR:  duplicate key value violates unique constraint "unique_pair_ix"
DETAIL:  Key (LEAST(thing_one, thing_two), GREATEST(thing_one, thing_two))=(1, 2) already exists.