SQL Stuff (Attempt #2)

postgres=# CREATE DATABASE test_guru;
CREATE DATABASE
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 int REFERENCES categories(id)
postgres(# );
CREATE TABLE
postgres=# CREATE TABLE questions (
postgres(# id serial PRIMARY KEY,
postgres(# body text,
postgres(# test_id int REFERENCES tests(id)
postgres(# );
CREATE TABLE
postgres=# INSERT INTO categories(title) VALUES
postgres-# ('Web Development'),
postgres-# ('General'),
postgres-# ('Machine Learning');
INSERT 0 3
postgres=# INSERT INTO tests(title, level, category_id) VALUES
postgres-# ('Ruby', 0, 2),
postgres-# ('Rails', 1, 1),
postgres-# ('JavaScript', 2, 1),
postgres-# ('Artificial Intelligence', 3, 3),
postgres-# ('Python', 0, 2);
INSERT 0 5
postgres=# INSERT INTO questions(body, test_id) VALUES
postgres-# ('Input method in ruby', 1),
postgres-# ('Default file extension for views', 2),
postgres-# ('Value to represent absence of value', 3),
postgres-# ('Acronym for Artificial Intelligence', 4),
postgres-# ('Name of a data type that stores key-value pairs', 5);
INSERT 0 5
postgres=# SELECT * FROM tests WHERE (level = 2) OR (level = 3);
 id |          title          | level | category_id 
----+-------------------------+-------+-------------
  3 | JavaScript              |     2 |           1
  4 | Artificial Intelligence |     3 |           3
(2 rows)

postgres=# SELECT * FROM questions WHERE (test_id = 2);
 id |               body               | test_id 
----+----------------------------------+---------
  2 | Default file extension for views |       2
(1 row)

postgres=# UPDATE tests SET title = 'Ruby on Rails', level = 2 WHERE (title = 'Rails');
UPDATE 1
postgres=# DELETE FROM questions WHERE (test_id = 4);
DELETE 1
postgres=# SELECT tests.title AS test_title, categories.title AS category_title
postgres-# FROM tests JOIN categories ON tests.category_id = categories.id;
       test_title        |  category_title  
-------------------------+------------------
 Ruby                    | General
 JavaScript              | Web Development
 Artificial Intelligence | Machine Learning
 Python                  | General
 Ruby on Rails           | Web Development
(5 rows)

postgres=# SELECT questions.body AS question_body, tests.title AS test_title
postgres-# FROM questions JOIN tests ON questions.test_id = tests.id;
                  question_body                  |  test_title   
-------------------------------------------------+---------------
 Input method in ruby                            | Ruby
 Default file extension for views                | Ruby on Rails
 Value to represent absence of value             | JavaScript
 Name of a data type that stores key-value pairs | Python
(4 rows)