create table users ( id int not null primary key auto_increment, nick varchar(150));create table session ( id int not null primary key auto_increment, idFrom int not null, idTo int not null, constraint fk_session_user_from foreign key (idFrom) references user(id), constraint fk_session_user_to foreign key (idTo) references user(id));create table messages ( id int not null primary key auto_increment, idSession int not null, message varchar(4000), time timestamp default current_time(), constraint fk_message_session_id foreign key (idSession) references session(id));