Are these two ERDs the same? Do they represent the same thing? [Weak entities and multi-valued attributes]

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.

It looks to me as though child_1 and child_2 are two instances of a single entity type.

If so, then you have a (second) relationship such that

1. Every parent must have one and only one favourite child
2. Every child may be the favourite of zero or more parents.

The (first) relationship being

1. A parent may have zero or more children (unless, of course you have defined "parent" such that a child is a pre-requisite of being a parent ;-))
2. A child must have exactly two parents (under basic human biology ;-))

Your first relationship may, of course, be different depending on the context.