Showing Revision 3 created 05/24/2016 by Udacity Robot.

1. Title:
   Foreign Keys - Intro to Relational Databases
2. Description:

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   In database terminology, a column
   with a references constraint on it,
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   is also called a foreign key.
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   Now this has always sounded like
   a bit of a weird term to me,
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   like it was the key to a lock
   from far across the sea, but
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   a foreign key is just a column or
   a set of columns in one table,
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   that uniquely identifies
   a row in another table.
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   It's possible for
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    a table to have two or more foreign
    keys, actually that's really common.
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    For instance, consider a database of
    students grades in various courses.
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    Here we've got a table of students
    names and their ID numbers.
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    And the ID is a primary key
    because names aren't unique.
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    And we have a table of courses, with
    course names and course IDs as well.
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    And then we have the grades table,
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    which has foreign keys
    into both of those tables.
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    Here are the same tables,
    but this time drawn out.
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    Usually a foreign key will point to
    the primary key of the referenced table.
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    That's because the whole point of
    a foreign key is to uniquely identify
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    a row in the referenced table, and
    the table's primary key does that.
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    Now here's a quiz.
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    These tables are for
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    a more complex forum app than
    the one you worked on last lesson.
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    This app allows users to log in and
    post comments, and
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    also vote on other people's posts.
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    Take a look at these tables, and see if
    you can figure out which columns should
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    be primary keys, and which columns
    should be foreign keys referencing them.
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    In the users table,
    we have a username or handle or
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    alias, and the user's full name.
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    In this posts table,
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    we have the post content, the author's
    username, and an ID number for the post.
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    And in the votes table,
    we have the id number of a post,
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    the username of somebody who
    voted on that post, and a 1 or
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    minus 1 for whether they liked it or
    disliked it.
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    So for each one of these columns, if it
    should be a primary key of that table,
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    put a P in that box.
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    If it should be a foreign key, put an F.
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    If it shouldn't be either one,
    leave the box empty.
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    Also, although there's no check boxes
    for them, think about if any of
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    these tables should have primary
    keys with more than one column.