Showing Revision 3 created 05/25/2016 by Udacity Robot.

1. Title:
   Related Tables - Intro to Relational Databases
2. Description:

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   So a database,
   will usually have several tables in it.

   ¶

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   Here's how we might start for
   our image voting app.
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   The first table represents pictures
   that people have uploaded of animals.
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   The second represents people's votes.
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   Fluffy, Monster, and
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   George are all animals,
   whose pictures someone has uploaded.
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   Because, we might have two
   animals with the same name.
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    We give each one a numeric id here.
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    The pictures tables says
    that Fluffy has id 1.
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    Monster has id 2.
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    George has id 3.
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    And so on.
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    The votes table says, which images
    have been displayed together for
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    voting, and
    which one the user picked as the cutest.
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    Here, the app displayed monster,
    ID 2, and George, ID 3,
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    and the user voted for George.
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    2 and 3 were matched up,
    and 3 was the winner.
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    Note that in the votes table,
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    the columns are called left,
    right, and winner.
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    But they match up, to the column
    called id in the pictures table.
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    You can read every row as a sentence.
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    In the pictures table, the sentences
    say Fluffy has id number 1, and
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    the filename fluffsocute.jpg,
    Monster has id number 2,
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    and filename monstie-basket.png and
    so forth.
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    In the votes table,
    the sentences are picture 2 and
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    picture 3, were displayed and
    the user voted for picture 3.
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    Picture 1 and
    picture 3 were displayed, and
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    the user voted for picture 1 and so on.
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    So this 3 and this 3, refer to the same
    thing, a cute critter with id number 3.
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    But they're in different tables, and
    the columns have different names,
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    because they play different
    roles in different sentences.
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    Over here we're saying,
    which picture has which id?
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    And over here we're saying, which ones
    have been displayed together, and
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    who got the vote?
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    So this is the kind of sentence that
    this table actually represents.
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    If you wanted to come up with sentences
    like Monster was shown to Fluffy, and
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    Fluffy got the vote instead
    of 2 was shown with 1,
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    and 1 got the vote for this row here.
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    We would have to connect rows from
    this table, with rows from that table.
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    That's something we can do
    with a database query as well.
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    Queries that do this are called joins,
    and
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    we'll be seeing a lot of
    them later on in the course.
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    For now, just remember that
    a value with the same meaning,
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    can occur in different tables and
    have different column names, and
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    that we can derive new tables by
    linking up existing tables using joins.