gistfile1.txt

postgres=# CREATE DATABASE test_guru;
CREATE DATABASE

postgres-# \c test_guru

test_guru=# CREATE TABLE categories(id serial PRIMARY KEY,title varchar(25));
CREATE TABLE
test_guru=# \dt
           List of relations
 Schema |    Name    | Type  |  Owner   
--------+------------+-------+----------
 public | categories | table | postgres
(1 row)

test_guru=# CREATE TABLE tests(id serial PRIMARY KEY,title varchar(100),level int,category_id int);
CREATE TABLE
test_guru=# \dt
           List of relations
 Schema |    Name    | Type  |  Owner   
--------+------------+-------+----------
 public | categories | table | postgres
 public | tests      | table | postgres
(2 rows)

test_guru=# CREATE TABLE questions(id serial PRIMARY KEY,body varchar(100),test_id int);
CREATE TABLE
test_guru=# \dt
           List of relations
 Schema |    Name    | Type  |  Owner   
--------+------------+-------+----------
 public | categories | table | postgres
 public | questions  | table | postgres
 public | tests      | table | postgres
(3 rows)

test_guru=# INSERT INTO categories(title) values ('category_one'),
test_guru-# ('category_two'),('category_three');
INSERT 0 3

test_guru=# INSERT INTO tests(title, level, category_id) VALUES
test_guru-# ('tests_one', 1, 1), ('tests_two', 2, 2),('tests_three', 1, 3),
test_guru-# ('tests_fore', 2, 1),('tests_five', 3, 3);
INSERT 0 5

test_guru=# INSERT INTO questions(body, test_id) VALUES
test_guru-# ('question_one', 5),('question_two', 4),
test_guru-# ('question_three', 3),('question_fore', 2),
test_guru-# ('question_five', 1);
INSERT 0 5

test_guru=# SELECT * FROM tests WHERE level IN (2,3);
 id |   title    | level | categories_id 
----+------------+-------+---------------
  2 | tests_two  |     2 |             2
  4 | tests_fore |     2 |             1
  5 | tests_five |     3 |             3
(3 rows)

test_guru=# SELECT * FROM questions WHERE test_id = 1;
 id |     body      | test_id 
----+---------------+---------
  5 | question_five |       1
(1 row)

test_guru=# UPDATE tests
test_guru-# SET title = 'tests_update',
test_guru-# level = level + 1
test_guru-# WHERE title = 'tests_fore';
UPDATE 1

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

test_guru=# SELECT t.title, c.title FROM categories c INNER JOIN tests t ON t.category_id = c.id;
    title     |     title      
--------------+----------------
 tests_one    | category_one
 tests_two    | category_two
 tests_three  | category_three
 tests_five   | category_three
 tests_update | category_one
(5 rows)

test_guru=# SELECT q.body, t.title FROM tests t INNER JOIN questions q ON q.test_id = t.id;
      body      |    title     
----------------+--------------
 question_one   | tests_five
 question_two   | tests_update
 question_three | tests_three
 question_fore  | tests_two
(4 rows)

здесь по аналогии с test_id стоит заменить на единственное число имя колонки

алиасы для столбцов не задал, Евгений писал про это