Added a solution for problem 14.4.

ch-14-databases/.gitignore

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+join_example.db

ch-14-databases/04-joins.py

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+# Explain the various types of database table joins.
+
+# In SQL, the JOIN keyword is used to combine records in tables according to
+# a specified field in each.  When the specified fields match, the records are
+# joined together.  The different types of joins (INNER, LEFT, RIGHT, and
+# OUTER) only differ in how they handle records that do not have a match in
+# the other table.
+
+# INNER joins ignore records that do not match.  OUTER joins include all records
+# whether or not they matched with a record from the other table.  LEFT joins
+# include records in the left table whether or not they match, but do not 
+# include unmatched records in the right table.  RIGHT joins are the reverse
+# of LEFT joins.  When a record is included that doe not match another record
+# NULL values are used where a matching records' values would otherwise be.
+
+# Consider the following two tables.  An inner join on the author's name
+# would contain four rows for the combinations of Randall Munroe's comics
+# and books and one row for Matthew Inman.  A left join with the Comic as the
+# first table would additionally return a row for Zach Weinersmith with NULL
+# values for the book.title, book.author, and book.year.  A full outer join
+# would contain six rows.
+
+# Comic Table
+# +=============+==================+=============================+
+# | Name        | Author           | URL                         |
+# +=============+==================+=============================+
+# | XKCD        | Randall Munroe   | https://www.xkcd.com/       |
+# +-------------+------------------+-----------------------------+
+# | What If?    | Randall Munroe   | https://what-if.xkcd.com/   |
+# +--------------+-----------------+-----------------------------+
+# | The Oatmeal | Matthew Inman    | http://theoatmeal.com/      |
+# +-------------+------------------+-----------------------------+
+# | SMBC        | Zach Weinersmith | http://www.smbc-comics.com/ |
+# +-------------+------------------+-----------------------------+
+
+# Book Table
+# +=================================================+================+======+
+# | Title                                           | Author         | Year |
+# +=================================================+================+======+
+# | What If?                                        | Randall Munroe | 2014 |
+# +-------------------------------------------------+----------------+------+
+# | Thing Explainer                                 | Randall Munroe | 2015 |
+# +-------------------------------------------------+----------------+------+
+# | How to Tell if Your Cat is Plotting to Kill You | Matthew Inman  | 2012 |
+# +-------------------------------------------------+----------------+------+
+# | The Hitchhiker's Guide to the Galaxy            | Douglas Adams  | 1978 |
+# +-------------------------------------------------+----------------+------+
+
+import unittest
+import sqlite3
+conn = sqlite3.connect('join-example.db')
+
+class Test(unittest.TestCase):
+  def setUp(self):
+    c = conn.cursor()
+    c.execute('DROP TABLE IF EXISTS Comic;')
+    c.execute('DROP TABLE IF EXISTS Book;')
+    c.execute('CREATE TABLE Comic(Name TEXT, Author TEXT, URL TEXT);')
+    c.execute('INSERT INTO Comic VALUES ("XKCD", "Randall Munroe", "https://www.xkcd.com/");')
+    c.execute('INSERT INTO Comic VALUES ("What If?", "Randall Munroe", "https://what-if.xkcd.com/");')
+    c.execute('INSERT INTO Comic VALUES ("The Oatmeal", "Matthew Inman", "http://theoatmeal.com/");')
+    c.execute('INSERT INTO Comic VALUES ("SMBC", "Zach Weinersmith", "http://www.smbc-comics.com/");')
+    c.execute('CREATE TABLE Book(Title TEXT, Author TEXT, Year INT);')
+    c.execute('INSERT INTO Book VALUES ("What If?", "Randall Munroe", 2014);')
+    c.execute('INSERT INTO Book VALUES ("Thing Explainer", "Randall Munroe", 2015);')
+    c.execute('INSERT INTO Book VALUES ("How to Tell if Your Cat is Plotting to Kill You", "Matthew Inman", 2012);')
+    c.execute('INSERT INTO Book VALUES ("The Hitchhiker\'s Guide to the Galaxy", "Douglas Adams", 1978);')
+    conn.commit()
+
+  def test_inner_join(self):
+    c = conn.cursor()
+    c.execute('SELECT Title, Book.Author, URL from Book INNER JOIN Comic ON Book.Author = Comic.Author;')
+    rows = c.fetchall()
+    self.assertEqual(len(rows), 5)
+    self.assertEqual(rows, [
+        (u"What If?", u"Randall Munroe", u"https://what-if.xkcd.com/"),
+        (u"What If?", u"Randall Munroe", u"https://www.xkcd.com/"),
+        (u"Thing Explainer", u"Randall Munroe", u"https://what-if.xkcd.com/"),
+        (u"Thing Explainer", u"Randall Munroe", u"https://www.xkcd.com/"),
+        (u"How to Tell if Your Cat is Plotting to Kill You", u"Matthew Inman", "http://theoatmeal.com/")])
+
+  def test_left_join(self):
+    c = conn.cursor()
+    c.execute('SELECT Title, Book.Author, URL from Book LEFT JOIN Comic ON Book.Author = Comic.Author;')
+    rows = c.fetchall()
+    self.assertEqual(len(rows), 6)
+    self.assertEqual(rows, [
+        (u"What If?", u"Randall Munroe", u"https://what-if.xkcd.com/"),
+        (u"What If?", u"Randall Munroe", u"https://www.xkcd.com/"),
+        (u"Thing Explainer", u"Randall Munroe", u"https://what-if.xkcd.com/"),
+        (u"Thing Explainer", u"Randall Munroe", u"https://www.xkcd.com/"),
+        (u"How to Tell if Your Cat is Plotting to Kill You", u"Matthew Inman", "http://theoatmeal.com/"),
+        (u"The Hitchhiker\'s Guide to the Galaxy", u"Douglas Adams", None)])
+
+if __name__ == "__main__":
+  unittest.main()
+
+