CREATE TABLE IF NOT EXISTS `books` ( `id` BIGINT UNSIGNED NOT NULL AUTO_INCREMENT, `name` TEXT NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO books (id, name) values (1, 'A'), (2, 'B'), (3, 'C') ;

CREATE TABLE IF NOT EXISTS `compare` ( `id_1` BIGINT UNSIGNED NOT NULL, `id_2` BIGINT UNSIGNED NOT NULL, PRIMARY KEY (`id_1`,`id_2`), FOREIGN KEY (`id_1`) REFERENCES books(`id`), FOREIGN KEY (`id_2`) REFERENCES books(`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

ALTER TABLE compare ADD small_id BIGINT AS (LEAST(id_1, id_2)) VIRTUAL, ADD big_id BIGINT AS (GREATEST(id_1, id_2)) VIRTUAL, ADD CONSTRAINT ids_unique_combination_uq UNIQUE (small_id, big_id);

INSERT INTO compare (id_1, id_2) values (1, 2) ;

select * from compare ;

INSERT INTO compare (id_1, id_2) values (2, 1) ;

INSERT INTO compare (id_1, id_2) values (2, 3), (3,2) ;

INSERT INTO compare (id_1, id_2) values (3,2) ;

select * from compare ;