Results 1 to 9 of 9
  1. #1
    Join Date
    Nov 2009
    Posts
    4

    Red face A little help on developing my first ERD

    Dear dBfriends,

    I am unable to solve the following problem which requires to draw an ER diagram that describes each of the following situations, assuming no further constraints hold:

    A university database contains information about professors (identified by social security number, or SSN) and courses (identified by courseid). Professors teach courses; each of the following situations concerns the Teaches relationship set.

    1. Professors can teach the same course in several semesters, and each offering must be recorded.
    2. Professors can teach the same course in several semesters, and only the most recent such offering needs to be recorded. (Assume this condition applies in all subsequent questions.)
    3. Every professor must teach some course.
    4. Every professor teaches exactly one course (no more, no less).
    5. Every professor teaches exactly one course (no more, no less), and every course must be taught by some professor.
    6. Finally, suppose that certain courses can be taught by a team of professors jointly, but it is possible that no one professor in a team can teach the course. Model this situation, introducing additional entity sets and relationship sets if necessary.

    I thank you in advance for any help.
    _mxm_

  2. #2
    Join Date
    Jul 2009
    Location
    Michigan
    Posts
    125
    We aren't going to do your homework for you, but we will help with questions. Which part are you having trouble with?

  3. #3
    Join Date
    Nov 2009
    Posts
    4
    Hello,

    my intent is not to have a homework already done, since I am not a young student I am starting now in studying DMBS and so I have no confidence to make ERD. That's all.
    As it probably has been recognized, the one posted is a well known exercise from one of the most authoritative book on market...
    The problem is easy: on a piece of sheet I down't know ho to write the diagram with RECTANGLES (entities), RHOMBUSES (relationships), connections and their cardinalities.
    I have joined on this forum hoping in a little help, maybe two or more examples to understand the way to proceed and that's all! On my side, this does not look as a so strange request...

    I've attempted to make my rough ER diagram, but unfortunately I don't know how to distinguish correctly point 1 from 2.

    The result is in the attached jpeg. Please, could anyone give to me some suggestion?

    _mxm_
    Attached Thumbnails Attached Thumbnails img1.jpg  
    Last edited by _maxim_; 11-17-09 at 04:40.

  4. #4
    Join Date
    Jul 2009
    Location
    Michigan
    Posts
    125
    I don't understand what they are getting at with point 1 and 2 either. They seem to be exact opposites of each other.

    With point 1 Semester would be an entity with relationships to Professors and Courses. With point 2 Semester could be ignored entirely since you only care about the current semester.

  5. #5
    Join Date
    Nov 2002
    Location
    Jersey
    Posts
    10,322
    Quote Originally Posted by _maxim_ View Post
    Dear dBfriends,
    1. Professors can teach the same course in several semesters, and each offering must be recorded.
    Prof Table, Course Table, Class Table (the day and time a course is offered, and the Prof teaching it)

    2. Professors can teach the same course in several semesters, and only the most recent such offering needs to be recorded.
    Marisa Tomei: "It's a bullsheet question"

    3. Every professor must teach some course.
    See Marisa

    4. Every professor teaches exactly one course (no more, no less).
    Well that's just nonsense...I can't address anymore

    5. Every professor teaches exactly one course (no more, no less), and every course must be taught by some professor.
    6. Finally, suppose that certain courses can be taught by a team of professors jointly, but it is possible that no one professor in a team can teach the course. Model this situation, introducing additional entity sets and relationship sets if necessary.

    I thank you in advance for any help.
    _mxm_


    Do YOU think ANY of this is a REAL world application?

    Most of the problem is the "Exercise" gives you stuff you would NEVER believe to be true

    Sounds like an MSCE exam

    my Advice is to build something a real world application would actually use

    Most of the stuff like "only 1", "can only teach" cann not be installed using a relational model, and must use either triggers or business rules in application code

    This is a very POOR exercise in teaching the relational model

    MOO
    Brett
    8-)

    It's a Great Day for America everybody!

    dbforums Yak CorralRadio 'Rita
    dbForums Member List
    I'm Good Once as I ever was

    The physical order of data in a database has no meaning.

  6. #6
    Join Date
    Nov 2009
    Posts
    4
    Hey, I never said that is a *REAL* world application, and, sincerely, I don't care at all! I want only start drawing simple ER diagrams and having a cross-check with persons supposed to be expert on this field.

    Damn! This is the book I'm reading: http://pages.cs.wisc.edu/~dbbook/images/book3ed.jpg

    Exercise 2.2 pg. 12 is the one on which I'm playing with: http://pages.cs.wisc.edu/%7Edbbook/o...ed-oddonly.pdf

    I am really disappointed by the answers I got on my first post!


    _mxm_

  7. #7
    Join Date
    Jul 2009
    Location
    Michigan
    Posts
    125
    I have tried to answer your question about points 1 and 2, but even after reading the online PDF I still can't. You are going to have to get a teacher or somebody to explain the difference between the first two points since they seem to contradict each other.

  8. #8
    Join Date
    Nov 2009
    Posts
    4
    Quote Originally Posted by MarkATrombley View Post
    I have tried to answer your question about points 1 and 2, but even after reading the online PDF I still can't. You are going to have to get a teacher or somebody to explain the difference between the first two points since they seem to contradict each other.
    I agree completely with you ;-) so, forget about the 2nd point and go ahead! I am trying to make a diagram for the 3rd point. Later on I will post it here. If you can have a look I'll be glad a lot!

    _mxm_

  9. #9
    Join Date
    Nov 2002
    Location
    Jersey
    Posts
    10,322
    Not sure if this an attempt to yell at us, but, did you read

    >>Most of the stuff like "only 1", "can only teach" cann not be installed using a relational model, and must use either triggers or business rules in application code

    These are not things that a constrained by a relation model

    Read this

    Normalization
    Brett
    8-)

    It's a Great Day for America everybody!

    dbforums Yak CorralRadio 'Rita
    dbForums Member List
    I'm Good Once as I ever was

    The physical order of data in a database has no meaning.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •