create table task (
    id number not null,
    title varchar (50),
    constraint tasks_pk primary key (id)
);

create table user_web (
    id number not null,
    name varchar(50),
    constraint user_id primary key (id)
);

create table solution (
    id number not null,
    student_id number not null,
    task_id number not null,
    sent_date date,
    CONSTRAINT solutions_pk PRIMARY KEY (id),
    CONSTRAINT fk_studentt_id
        FOREIGN key (student_id)
        references user_web (id),
   CONSTRAINT fk_taskk_id
        FOREIGN key (task_id)
        references task (id)
);

create table grades (
    solution_id number not null,
    tutor_id number not null,
    grade number,
    grade_date date,
    CONSTRAINT fk_solutions_id
        FOREIGN key (solution_id)
        references solution (id),
   CONSTRAINT fk_tutor_id
        FOREIGN key (tutor_id)
        references user_web (id)
);

INSERT INTO user_web (ID, NAME) VALUES (1, 'Danil');
INSERT INTO user_web (ID, NAME) VALUES (2, 'Kiril');
INSERT INTO user_web (ID, NAME) VALUES (3, 'Maxim');

insert into task (id, title) values (0, 'Rp_01');
insert into task (id, title) values (1, 'Rp_02');
insert into task (id, title) values (2, 'Rp_03');
insert into task (id, title) values (3, 'Rp_04');
insert into task (id, title) values (4, 'Rp_05');

insert into user_web (id, name) VALUES (4, 'Olga Kovalenko');
insert into user_web (id, name) VALUES (5, 'Oleg Kuzikov');

insert into solution (id, student_id, task_id, sent_date)
VALUES (1, 3, 0, to_date('2022-04-23','YYYY-MM-DD'));

insert into solution (id, student_id, task_id, sent_date)
VALUES (2, 3, 1, to_date('2022-04-25','YYYY-MM-DD'));

insert into solution (id, student_id, task_id, sent_date)
VALUES (3, 1, 0, to_date('2022-04-26','YYYY-MM-DD'));

insert into solution (id, student_id, task_id, sent_date)
VALUES (4, 1, 1, to_date('2022-04-27','YYYY-MM-DD'));

insert into grades (solution_id, tutor_id, grade, grade_date)
VALUES (1, 4, 50, to_date('2022-04-27','YYYY-MM-DD'));

insert into grades (solution_id, tutor_id, grade, grade_date)
VALUES (2, 4, 80, to_date('2022-04-29','YYYY-MM-DD'));

insert into grades (solution_id, tutor_id, grade, grade_date)
VALUES (3, 5, 40, to_date('2022-04-28','YYYY-MM-DD'));

insert into grades (solution_id, tutor_id, grade, grade_date)
VALUES (4, 5, 90, to_date('2022-04-30','YYYY-MM-DD'));

insert into grades (solution_id, tutor_id, grade, grade_date)
VALUES (0, 4, 60, to_date('2022-04-25','YYYY-MM-DD'));