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  1. #1
    Join Date
    Dec 2004
    Posts
    6

    Basic ERD sematics question

    The "normal" appearance of an ERD, like I know it, can be seen on here:
    http://www.utexas.edu/its/windows/da...m/erintro.html
    on the bottom of the page, right above "Summary"

    Now I stumbled several times over another notation, which I will try to illustrate:

    country--(1,n)---<is in>---(1,1)--city

    where <> is kind of a rhombus, representing a "relationship-type". The cardinalities in parenthesises representing the (min,max). So one could think that, one city is in "n", where n is at least 1, countries.
    That, I think, is just plain wrong. It is of course just the other way round. "n", where n is at least 1, cities are in one country (asuming that a country has at least one city, which is quite a safe asumption).
    What am I misinterpreting?
    Is this notation read backwards?
    How would one design the city/country example in a "normal" ERD (with the first notation) ? Would it be something like this: country-1-----n-<city ?

  2. #2
    Join Date
    Dec 2003
    Location
    Canada
    Posts
    710
    There's nothing wrong with the example the left side shows the relationship of one country to many cities, the right side shows the relationship of one city to one country. In otherwords a country can have many cities, but a city of a certain name can exist in one and only one country.

  3. #3
    Join Date
    Apr 2002
    Location
    Toronto, Canada
    Posts
    20,002
    no, not "a city of a certain name"

    rather, a city of a certain identity

    what constitutes unique identity for a city is a different question, but i can tell you it is not the city name

    toronto is in canada and it is also in the united states more than once
    rudy.ca | @rudydotca
    Buy my SitePoint book: Simply SQL

  4. #4
    Join Date
    Dec 2004
    Posts
    6
    Ok, thanks for the replies.

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