SQL2

postgres=# CREATE DATABASE test_guru;
CREATE DATABASE
postgres=# CREATE TABLE categories (
postgres(#  id serial PRIMARY KEY,
postgres(#  title varchar(40)
postgres(# );
CREATE TABLE
postgres=# CREATE TABLE tests (
postgres(#  id serial PRIMARY KEY,
postgres(#  title varchar(40),
postgres(#  level int,
postgres(#  category_id int
postgres(# );
CREATE TABLE
postgres=# CREATE TABLE questions (
postgres(#  id serial PRIMARY KEY,
postgres(#  body varchar(50),
postgres(#  test_id int
postgres(# );
CREATE TABLE
postgres=# INSERT INTO categories(title) VALUES
postgres-#  ('Frontend'),
postgres-#  ('Backend'),
postgres-#  ('Fullstack');
INSERT 0 3
postgres=# SELECT*
postgres-# FROM categories;
 id |   title
----+-----------
  1 | Frontend
  2 | Backend
  3 | Fullstack
(3 строки)

postgres=# INSERT INTO tests(title, level, category_id) VALUES
postgres-#  ('Ruby', 2, 2),
postgres-#  ('HTML', 1, 1),
postgres-#  ('CSS', 2, 1),
postgres-#  ('SQL', 3, 2),
postgres-#  ('RoR', 3, 3);
INSERT 0 5
postgres=# SELECT*
postgres-# FROM tests;
 id | title | level | category_id
----+-------+-------+-------------
  1 | Ruby  |     2 |           2
  2 | HTML  |     1 |           1
  3 | CSS   |     2 |           1
  4 | SQL   |     3 |           2
  5 | RoR   |     3 |           3
(5 строк)

postgres=# INSERT INTO questions(body, test_id) VALUES
postgres-#  ('Why?', 2),
postgres-#  ('Why not?', 1),
postgres-#  ('Where?', 4),
postgres-#  ('Who?', 2),
postgres-#  ('How?', 3);
INSERT 0 5
postgres=# SELECT*
postgres-# FROM questions;
 id |   body   | test_id
----+----------+---------
  1 | Why?     |       2
  2 | Why not? |       1
  3 | Where?   |       4
  4 | Who?     |       2
  5 | How?     |       3
(5 строк)

postgres=# SELECT* FROM tests WHERE level>1;
 id | title | level | category_id
----+-------+-------+-------------
  1 | Ruby  |     2 |           2
  3 | CSS   |     2 |           1
  4 | SQL   |     3 |           2
  5 | RoR   |     3 |           3
(4 строки)

postgres=# SELECT*
postgres-# FROM questions
postgres-# WHERE test_id = 2;
 id | body | test_id
----+------+---------
  1 | Why? |       2
  4 | Who? |       2
(2 строки)

postgres=# UPDATE tests
postgres-# SET title = 'HAML', level = 3
postgres-# WHERE title = 'HTML';
UPDATE 1
postgres=# SELECT*
postgres-# FROM tests;
 id | title | level | category_id
----+-------+-------+-------------
  1 | Ruby  |     2 |           2
  3 | CSS   |     2 |           1
  4 | SQL   |     3 |           2
  5 | RoR   |     3 |           3
  2 | HAML  |     3 |           1
(5 строк)

postgres=# DELETE
postgres-# FROM questions
postgres-# WHERE test_id = 4;
DELETE 1
postgres=# SELECT*
postgres-# FROM questions;
 id |   body   | test_id
----+----------+---------
  1 | Why?     |       2
  2 | Why not? |       1
  4 | Who?     |       2
  5 | How?     |       3
(4 строки)

postgres=# SELECT tests.title, categories.title
postgres-# FROM tests
postgres-# JOIN categories
postgres-# ON tests.category_id = categories.id;
 title |   title
-------+-----------
 Ruby  | Backend
 CSS   | Frontend
 SQL   | Backend
 RoR   | Fullstack
 HAML  | Frontend
(5 строк)

postgres=# SELECT questions.body, tests.title
postgres-# FROM questions
postgres-# JOIN tests
postgres-# ON questions.test_id = tests.id;
   body   | title
----------+-------
 Why?     | HAML
 Why not? | Ruby
 Who?     | HAML
 How?     | CSS
(4 строки)

https://gist.github.com/Sandstorm750/8f7c3f058a97c903a8119c311223b377#file-sql2-L118
Здесь удобно использовать алиасы, чтобы было ясно где чей заголовок.