postgres=# CREATE TABLE categories (
postgres(# id serial primary key,
postgres(# title varchar(50)
postgres(# );
CREATE TABLE


postgres=# CREATE TABLE tests (
postgres(# id serial primary key,
postgres(# title varchar(50),
postgres(# level int,
postgres(# category_id serial references categories(id)
postgres(# );
CREATE TABLE

postgres=#  CREATE TABLE questions (
postgres(# id serial primary key,
postgres(# body varchar(50),
postgres(# test_id serial references tests(id)
postgres(# );
CREATE TABLE

postgres=# INSERT INTO categories(title) VALUES
postgres-# ('geography'),
postgres-# ('math'),
postgres-# ('history');
INSERT 0 3

postgres=# INSERT INTO tests (title, level, category_id) VALUES
postgres-# ('first test', 5, 2),
postgres-# ('second test', 3, 2),
postgres-# ('third test', 2,  3),
postgres-# ('fourth test', 9, 3),
postgres-# ('fifth test', 0, 1);
INSERT 0 5

postgres=# INSERT INTO questions (body, test_id) VALUES
postgres-# ('who is the creater of Ruby?', 3),
postgres-# ('what is 2+2?', 2),
postgres-# ('when was the Kennedy killed?', 4),
postgres-# ('who is the current world football champion?', 4),
postgres-# ('what is the capital of Australia?', 5);
INSERT 0 5

postgres=# SELECT *
postgres-# FROM tests
postgres-# WHERE level = 3 ;
 id |    title    | level | category_id
----+-------------+-------+-------------
  2 | second test |     3 |           2
(1 row)

postgres=# SELECT *
postgres-# FROM tests
postgres-# WHERE level = 2;
 id |   title    | level | category_id
----+------------+-------+-------------
  3 | third test |     2 |           3
(1 row)

postgres=# SELECT *
postgres-# FROM questions
postgres-# WHERE test_id = 4;
 id |                    body                     | test_id
----+---------------------------------------------+---------
  3 | when was the Kennedy killed?                |       4
  4 | who is the current world football champion? |       4
(2 rows)

postgres=# UPDATE tests
postgres-# SET title = 'sixth test', level = 78
postgres-# WHERE title = 'first test';
UPDATE 1

postgres=# DELETE FROM questions
postgres-# WHERE test_id = 4;
DELETE 2

postgres=# SELECT  tests.title, categories.title
postgres-# FROM tests INNER JOIN categories ON tests.category_id = categories.id;
    title    |   title
-------------+-----------
 second test | math
 third test  | history
 fourth test | history
 fifth test  | geography
 sixth test  | math
(5 rows)

postgres=# SELECT questions.body,  tests.title
postgres-# FROM questions INNER JOIN tests ON tests.id = questions.test_id ;
               body                |    title
-----------------------------------+-------------
 who is the creater of Ruby?       | third test
 what is 2+2?                      | second test
 what is the capital of Australia? | fifth test
(3 rows)