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Information about the same thing, from different point of view (need)

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• Dictatical comment
  • Comment:

    In the next few webpages, I will use a few colloquial terms to make things easier to understand: Presentation format              Stored format These terms are actually called: External schema              Conceptual schema But I think you will understand material better if I used the colloquail erms

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• Information seen by different users

  • Fact:

    Different group of users can have different information requirements on the same object

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  • Example: suppose we have 3 different applications on (same) students group

    Grade report Financial record           Health report

  • Sample of data file for each application:

    Grade Report Financial record (other files) StuID Name Course Grd StuID Name Due +-----+----------+-------+----+ +-----+----------+------+ ..... | 123 | John Doe | CS171 | A | | 007 | James Bo | 2000 | +-----+----------+-------+----+ +-----+----------+------+ | 123 | John Doe | CS255 | B | | 123 | John Doe | 1500 | +-----+----------+-------+----+ +-----+----------+------+ | 007 | James Bo | CS170 | A | +-----+----------+-------+----+ | 007 | James Bo | CS171 | B | +-----+----------+-------+----+

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• The presentation format of information
  • Presentation format of information:

    Presentation of the information = the format of information that a (class of) user sees (= uses) in their application (Note: the format presented (seen by) to the user must have all the necessary information !!!)

    Example:

    The users of the "Grade Report" application must have the following information to their disposal: Student ID Student name The courses that the student has taken The grade that the student has earn for each course Therefore, an appropriate record format that can provide the required information to the "Grade Report" application is as follows: StuID Name Course Grd +-----+----------+-------+----+ | 123 | John Doe | CS171 | A | +-----+----------+-------+----+ | 123 | John Doe | CS255 | B | +-----+----------+-------+----+ | 007 | James Bo | CS170 | A | +-----+----------+-------+----+ | 007 | James Bo | CS171 | B | +-----+----------+-------+----+

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• Stored record format
  • Stored record format:

    Stored record format = the record structure of the data file stored on disk

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• Presentation format vs. stored format
  • Important fact:

    The format that you use to store the data may be different from the format that you present the data

    Example:

    Suppose we store the student information as follows: StuID Name Course Grd due +-----+----------+-------+----+------+ | 123 | John Doe | CS171 | A | 1500 | +-----+----------+-------+----+------+ | 123 | John Doe | CS255 | B | 1500 | +-----+----------+-------+----+------+ | 007 | James Bo | CS170 | A | 2000 | +-----+----------+-------+----+------+ | 007 | James Bo | CS171 | B | 2000 | +-----+----------+-------+----+------+ Then we may choose to present the stored information as: StuID Name Course Grd +-----+----------+-------+----+ | 123 | John Doe | CS171 | A | +-----+----------+-------+----+ | 123 | John Doe | CS255 | B | +-----+----------+-------+----+ | 007 | James Bo | CS170 | A | +-----+----------+-------+----+ | 007 | James Bo | CS171 | B | +-----+----------+-------+----+ by not displaying the due data field