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  1. #1
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    Apr 2012
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    E-R diagram for a university registration office

    I came across this question in a uni assignment of mine. Anyhow I found answers for it in the internet. But I cant understand some parts of it. This is the question

    Construct an E-R diagram for a university registration office maintains data about the following entities:
     Courses, including number, title, credits, syllabus, and prerequisites;
     Course_offerings, including course number, year, semester, section number, instructor(s), timings, and classroom;
     Students, including student-id, name, and program
     Instructors, including identification number, name, department, and title.

    The enrollment of students in courses and grades awarded to students in each course they are enrolled for must be appropriately modeled. State all assumptions that you make about the mapping constraints.

    Extend the ER diagram to record the marks that students get in different exams of different course offerings.

    Answer 1

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    question 1 - why can't we add "grade" attribute to the student entity ?
    question 2 - I cant understand what they have down with prerequisite attribute.cant we just add it as a multivalued attribute ?

    Answer 2

    The assumptions made are :
    a. a class meets only at one particular place and time. This E-R diagram cannot
    model a class meeting at different places at different times.
    b. There is no guarantee that the database does not have two classes meeting
    at the same place and time.

    question 1 - can you please explain these assumptions to me ?



    Answer 3

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    question 1 - in the question, it says to to extend the previous diagram. so don't we have to include other entities and previous relationships ?
    question 2 - cant we add marks attribute to the student entity

    can you please help me with these questions
    Last edited by vpowerc; 04-15-12 at 09:04.

  2. #2
    Join Date
    May 2008
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    277
    Quote Originally Posted by vpowerc View Post
    question 1 - why can't we add "grade" attribute to the student entity ?
    Think about how you would record a student that progresses through multiple grades. Take a look at your entities, come up with some sample data, and then enter that data into each entity as if you had actually created them as tables in your database.

    I cant understand what they have down with prerequisite attribute.cant we just add it as a multivalued attribute ?
    This would depend on whether or not a course has multiple prerequisites. But regardless, multivalued attributes can't be represented directly in a relational model. Take a look at this. Now consider that the prerequisite for a couse is simply another course, which is a recursive relationship.

    The assumptions made are :
    a. a class meets only at one particular place and time. This E-R diagram cannot
    model a class meeting at different places at different times.
    b. There is no guarantee that the database does not have two classes meeting
    at the same place and time.

    question 1 - can you please explain these assumptions to me ?
    Again, come up with some sample data for these situations and try to enter it into the entities you have. This should hopefully help you understand the limitations of the model.

    in the question, it says to to extend the previous diagram. so don't we have to include other entities and previous relationships ?
    I'm not sure what you're asking, but the 2nd drawing "extends" the first, which means it adds additional entities and relationships to the 1st diagram, but doesn't show the entire original diagram, only the new stuff. Based on the entities listed in the 2nd diagram, you should be able to figure out where it fits into the 1st diagram.

    cant we add marks attribute to the student entity
    Again, think about how you would record a student that receives marks for multiple classes.

  3. #3
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    Apr 2012
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    Quote Originally Posted by futurity View Post



    Again, come up with some sample data for these situations and try to enter it into the entities you have. This should hopefully help you understand the limitations of the model.


    Thank you very much for your explanations
    what is a class meeting mentioned in the assumptions ?
    what is mean by the classes ? I cant find a class in the diagram

  4. #4
    Join Date
    May 2008
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    277
    Quote Originally Posted by vpowerc View Post
    what is a class meeting mentioned in the assumptions ?
    what is mean by the classes ? I cant find a class in the diagram
    You're gonna have to use a little critical thought here. Not everything will always have the same name (although it would certainly behoove you to use consistent terminology to prevent confusion), especially if you're going to use 3rd-party data models to kickstart your own. Take a look at the entities and their attributes to determine which one could be the same thing as a "class": which one includes attributes that describe a meeting place and meeting time, and has a name that could be a reasonable synonym for "class"?

  5. #5
    Join Date
    Apr 2012
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    5

    difference between normalization and generalization

    can you tell me, what is the difference between normalization and generalization

  6. #6
    Join Date
    May 2008
    Posts
    277
    Generalization uses supertypes and subtypes to model similarities between different but related entities. It's what your assignment about putting books, CDs, and cassettes in a shopping cart is about. Here's a good blog article about it.

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