1. С помощью SQL (DDL):

CREATE DATABASE test_guru;

CREATE TABLE categories (
  id serial PRIMARY KEY,
  title varchar(50)
);

CREATE TABLE tests (
  id serial PRIMARY KEY,
  title varchar(50),
  level int, 
  category_id int
);

CREATE TABLE questions (
  id serial PRIMARY KEY,
  body varchar(200),
  test_id int
);

2. Выполните операции CRUD, JOIN:

INSERT INTO categories (title) VALUES
('Frontend'),
('Backend'),
('Machine learning');

INSERT INTO tests(title, level, category_id) VALUES
('Ruby', 1, 2),
('Ruby on Rails', 2, 2),
('JavaScript', 1, 1),
('Python', 3, 2),
('CSS', 1, NULL);

INSERT INTO questions(body, test_id) VALUES
('What is Ruby programming language?', 1),
('Why Ruby is known as a language of flexibility?', 1),
('Explain some differences between Ruby and Python.', 4),
('What are JavaScript Data Types?', 3),
('What is the use of isNaN function?', 3);

postgres=# SELECT * FROM tests WHERE level in (2, 3);
 id |     title     | level | category_id 
----+---------------+-------+-------------
  2 | Ruby on Rails |     2 |           2
  4 | Python        |     3 |           2
(2 rows)

postgres=# SELECT q.body FROM tests t
postgres-# JOIN questions q ON t.id = q.test_id
postgres-# WHERE t.id = 1;
                      body                       
-------------------------------------------------
 What is Ruby programming language?
 Why Ruby is known as a language of flexibility?
(2 rows)

postgres=# UPDATE tests SET title = 'RoR', level = 3 WHERE id = 7;
UPDATE 1

postgres=# DELETE FROM questions WHERE test_id = 3;
DELETE 2

postgres=# SELECT t.title as test, c.title as category FROM tests t
postgres-# JOIN categories c ON c.id = t.category_id;
    test    | category 
------------+----------
 Ruby       | Backend
 JavaScript | Frontend
 Python     | Backend
 RoR        | Backend
(4 rows)

postgres=# SELECT t.title as test, q.body as question FROM tests t
JOIN questions q ON t.id = q.test_id;
    test    |                     question                      
------------+---------------------------------------------------
 Ruby       | Why Ruby is known as a language of flexibility?
 JavaScript | What is the use of isNaN function?
 JavaScript | What are JavaScript Data Types?
 Python     | Explain some differences between Ruby and Python.
(4 rows)

https://gist.github.com/Sophiliya/0cf4bdddaa41597f50efccd635ef9e72#file-1-sql-ddl-L12 и https://gist.github.com/Sophiliya/0cf4bdddaa41597f50efccd635ef9e72#file-1-sql-ddl-L18 надо указать что эти колонки являются внешними ключами