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The objective is to design an Entity-Relationship Diagram for a company that has addresses, telephones, emails and representants. Here's the list of the constraints:

  • a company has one or more telephones
  • a company has one or more e-mails
  • a company has one or more address
  • an address has one or more representants
  • a representant has one or more telephones
  • a representant has one or more e-mails
  • if a representant is removed, then it must be saved in a list "last representants".

Other constraints are (hopefully) represented in the diagram, using the following notation:

  • circle: attribute
  • double-lined rectangle: weak entities
  • double-line: total participation
  • line ending with an arrow: cardinality 1:N (arrow points to the "1")

Entity Relationship Diagram

Entity-Relationship Diagram


I designed that diagram. I'm sure there are some bad design points, because I'm a beginner, and I would like to hear your opinions, suggestions and criticisms.

More information

(based on the answer by miracle173)

English is not my native language, I should've told that in my question, so please excuse my poor word picking. By representant I mean a person that will speak on behalf of the company. With telephones I mean one or more phone numbers. With emails I mean one or more email addresses.

Here is why I chose telephone and email as being weak entities: when a company is removed, I want the list of telephones numbers and the list of emails addresses associated with that company to be also removed. Did I achieve that goal with the use of weak entity?

An address of a company can have more than one representant, but the representant can only work for one company at a time (hence the restriction in my diagram). I agree, some singular and some plural is bad practice. I will fix that.

Changing the relation to more precise verb, like "represented by" (instead of "an address has representants"): from top to bottom, one can read "An address is represented by representants"; from bottom to top, it gives incorrect reading ("A representant is represented by a company"), when it should be "A representant represents a company". So I think it makes difference the reading direction.

What is wanted with "it must be saved" is: to have a short list, like a small history, of the last representants that worked for the company. Example: If John - which is a representant of Miracle's Corporation - suddenly is fired from the company, he should be removed from the Representants list (along with his phone numbers and email addresses), but it is desired to keep him in that short history list after his deletion from Representants.

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I am not sure if this is the right place to pose such questions. But here some remarks and questions anyway. They are too long for a comment.

  • I don't understand the word representant. I did not find it at http://dict.leo.org/. Dou you mean representative?

  • With telephones you mean phone numbers and with emails you mean email adresses?

  • Why is telephone a weak entity? What is its identifying entity? For me it is not a weak entity. A phone extension may be a weak entity. The same holds for email and address.
  • can a representant have more then one address? This is not excluded by the text but it is excluded by the ER diagtram.

  • Why do you name some entities as singular and some as plural? I think a consisten notation will be more clear.

  • Why do one need a reading direction? What is the difference if one reads the diagram from bottom to top? I see no difference.

  • "if a representant is removed it must be saved in a list" That does not describe any relationships between a removed representant and the remaining entities. "it must be saved" is a term of computer programming and not of the business domain. It is not clear to me what you want but using an attribute to a relation may be not the appropriate solution. From your drawing I conclude a telephone is assigned to a representant. If the representant is removed the removed representant has a telephone too? If yes then this telephone cannot be used by then new representant for this address because a telephone has only one representant assigned.

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Using the name "has" for every relationsip is a problem. For example, the link between Company and Address - is that the mailing address, delivery address, registered headquarters etc. By using such a weak word you obscure these differences which may cause ambiguity and data inconsistencies.

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