create table `test`.`user_relationship`(
        `first_user_id` int default '' not null comment 'ID of first user.',
       `second_user_id` int default '' not null comment 'ID of second user.',
        primary key (`first_user_id`,`second_user_id`)
    );

    alter table `test`.`user_relationship`  
        add index `second_user_fk`(`second_user_id`), 
        add constraint `second_user_fk` 
        foreign key (`second_user_id`) 
        references `test`.`user`(`ID`);
    alter table `test`.`user_relationship`  
        add index `first_user_fk`(`first_user_id`), 
        add constraint `first_user_fk` 
        foreign key (`first_user_id`) 
        references `test`.`user`(`ID`);
    create unique index `PRIMARY` on `test`.`user_relationship`(`first_user_id`,`second_user_id`);
    create index `first_user_fk` on `test`.`user_relationship`(`first_user_id`);
    create index `second_user_fk` on `test`.`user_relationship`(`second_user_id`);