CREATE TABLE T_USER (
    username VARCHAR( 40 ) not null primary key,
    pwd_digest VARCHAR( 255 ),
    last_name VARCHAR( 128 ),
    first_name VARCHAR( 128 )
);

CREATE TABLE T_GUESTBOOK (
    bookname VARCHAR( 40 ) not null primary key,
    description VARCHAR( 255 ),
    display_order INTEGER
);

CREATE TABLE T_MODERATOR (
    id BIGINT not null GENERATED ALWAYS AS IDENTITY,
    username VARCHAR( 40 ) not null,
    guestbook_name VARCHAR( 40 ) not null,

    CONSTRAINT pk_moderator PRIMARY KEY (id),
    CONSTRAINT fk_user FOREIGN KEY (username) REFERENCES t_guestbook(bookname) ON DELETE CASCADE,
    CONSTRAINT fk_guestbook_mod FOREIGN KEY (guestbook_name) REFERENCES T_GUESTBOOK(bookname) ON DELETE CASCADE
);

INSERT INTO T_USER (username, pwd_digest) VALUES ('root', '11886-2826124110125-98-95-10564-315099-119-16-65-72065');
INSERT INTO T_GUESTBOOK (bookname, description, display_order) VALUES ('gbsystem', 'Пример',  1 );
INSERT INTO T_MODERATOR (username, guestbook_name) VALUES ('root', 'gbsystem');

Код ошибки -1, положение SQL 23503: INSERT в таблице 'T_MODERATOR' вызывает нарушение ограничения чужого ключа 'FK_USER' для ключа (root).  Произведен откат оператора.