SQL

1.)

test_guru=# CREATE TABLE categories (
test_guru(# id SERIAL PRIMARY KEY,
test_guru(# title TEXT
);
CREATE TABLE



test_guru=# CREATE TABLE tests
test_guru(# id SERIAL PRIMARY KEY,
test_guru(# title TEXT,
test_guru(# level INTEGER,
test_guru(# category_id INTEGER REFERENCES categories(id)
test_guru(# );
CREATE TABLE



test_guru=# CREATE TABLE questions (
test_guru(# id SERIAL PRIMARY KEY,
body TEXT,
test_id INTEGER REFERENCES test(id)
);
CREATE TABLE



----------------------------------------------------------------------
2.)

test_guru=# INSERT INTO categories VALUES ('Ruby');
INSERT 0 1
test_guru=# INSERT INTO categories VALUES ('Rails');
INSERT 0 1
test_guru=# INSERT INTO categories VALUES ('Rspec');
INSERT 0 1
test_guru=#



test_guru=# INSERT INTO tests (title, level, category_id) VALUES ('Hashes', 1, 1);
INSERT 0 1
test_guru=# INSERT INTO tests (title, level, category_id) VALUES ('Classes', 3, 1);
INSERT 0 1
test_guru=# INSERT INTO tests (title, level, category_id) VALUES ('Metaprogramming', 6, 1);
INSERT 0 1
test_guru=# INSERT INTO tests (title, level, category_id) VALUES ('Rails routing', 1, 2);
INSERT 0 1
test_guru=# INSERT INTO tests (title, level, category_id) VALUES ('Rspec essentials', 2, 3);
INSERT 0 1
test_guru=#



test_guru=# INSERT INTO questions (body, test_id) VALUES ('How to declare a Hash?', 1)
test_guru-# ;
INSERT 0 1
test_guru=# INSERT INTO questions (body, test_id) VALUES ('How to declare a Class?', 2);
INSERT 0 1
test_guru=# INSERT INTO questions (body, test_id) VALUES ('How to call private method outside of a class?', 3);
INSERT 0 1
test_guru=# INSERT INTO questions (body, test_id) VALUES ('Where application routes is defined?', 4);
INSERT 0 1
test_guru=# INSERT INTO questions (body, test_id) VALUES ('What is a RSpec?', 5);
INSERT 0 1
test_guru=#


test_guru=# SELECT * FROM tests WHERE level IN (2, 3);
      title       | level | category_id | id
------------------+-------+-------------+----
 Classes          |     3 |           1 |  2
 Rspec essentials |     2 |           3 |  5
 Work with Hashes |     2 |           1 |  1
(3 rows)



test_guru=# SELECT * FROM questions WHERE test_id = 2
test_guru-# ;
          body           | test_id | id
-------------------------+---------+----
 How to declare a Class? |       2 |  2
(1 row)



test_guru=# UPDATE tests SET title = 'Work with Hashes', level = 2 WHERE title = 'Hashes';
UPDATE 1
test_guru=#



test_guru=# DELETE FROM questions WHERE test_id = 2;
DELETE 1
test_guru=#



test_guru=# SELECT tests.title, categories.title AS category_title FROM tests INNER JOIN categories
ON tests.category_id = categories.id
;
      title       | category_title
------------------+----------------
 Classes          | Ruby
 Metaprogramming  | Ruby
 Rails routing    | Rails
 Rspec essentials | Rspec
 Work with Hashes | Ruby
(5 rows)



test_guru=# SELECT questions.body AS question, tests.title AS test_title FROM questions INNER JOIN tests ON tests.id = questions.test_id
;
                    question                    |    test_title
------------------------------------------------+------------------
 How to declare a Hash?                         | Work with Hashes
 How to call private method outside of a class? | Metaprogramming
 Where application routes is defined?           | Rails routing
 What is a RSpec?                               | Rspec essentials
(4 rows)

тут в случае, если таблица будет использоваться как внешний ключ, стоит сразу это указать category_id INTEGER REFERENCES categories(id)

тут level in (2,3)

тут необходимо выбрать только названия тестов и категорий. так же посмотри про alias, т.к. у тебя эти столбцы имеют одинаковое название
тут здесь тоже должна быть не полная выборка