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  1. #1
    Join Date
    Aug 2008
    Location
    Australia
    Posts
    23

    Relational Algebra

    I am trying to learn relational algebra. Take the following equation;

    σ F ( r ) = { t ∣ t ∈ r ∧ F ( t ) } σ

    What does "∣" represent in this equation?
    What does "∧" represent?
    What do the braces { } represent?
    What do the brackets ( ) represent?
    What would be your "plain english" interpretation of this equation?
    Here is the Web page that is the source of this equation.
    Thanks.

  2. #2
    Join Date
    Dec 2007
    Location
    London, UK
    Posts
    741
    ∧ means "AND".
    t means "tuple t"
    Brackets and braces are just parentheses.
    In English "For all tuples t where t is a member of r AND F(t) is TRUE". This is a definition of restiction (σ) which they call "select".

    You should find a textbook with clearer explanations. "The Alice Book" is a classic (http://www.amazon.com/Foundations-Da...dp/0201537710/) but it is VERY heavy on the maths. A more accessible book is Chris Date's "Introduction to Database Systems".

  3. #3
    Join Date
    Apr 2002
    Location
    Toronto, Canada
    Posts
    20,002
    Quote Originally Posted by dportas
    A more accessible book is Chris Date's "Introduction to Database Systems".
    i guess you didn't see enuenu's other thread, then

    rudy.ca | @rudydotca
    Buy my SitePoint book: Simply SQL

  4. #4
    Join Date
    Dec 2007
    Location
    London, UK
    Posts
    741
    Seen it now, thanks.

  5. #5
    Join Date
    Aug 2008
    Location
    Australia
    Posts
    23
    Thanks all. I will check the text out. My baptism of fire is cooling a little now. It has been a while since I did any real maths.

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