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I am trying to create a conceptual data model using PowerDesigner, I found a conflie between two methods:

  1. Create two entities and relations between them
  2. Create an association and three entities and two relationships.

knowing that both methods give the same result on the generation of physical data model

both methods are illustrated by the following picture enter image description here

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    I think they come from different "methodology", E/R vs. (French) MERISE, so it may depends what's you're most used to. Association can have attributes, which makes some many-to-many easier to represent... – pascal Sep 06 '13 at 13:46

2 Answers2

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Associations and Relationships are different conceptual terms in Power-designer.

An Association is a relation between two or more entities.
An Association can have attributes.

A Relationship is a relation between two entities.
A Relationship has no attributes.

Association is more general form of Relationship.

For example:
Consider designing of a many to many relation between CustomerEntity and CreditGroupEntity
Designing the relation with Relationship will create a third table just contaning CustomerEntity primary-key and CreditGroupEntity primary-key.
Designing the relation with Association and adding a assign-date attribute to the association will create a third table contaning CustomerEntity primary-key, CreditGroupEntity primary-key and assign-date fields.

The sample you have used contains 0-n cardinal with no attributes on association, so the difference can't be seen.

Generally Association is used when the relation itself is an entity having attributes.
A relation, having no attributes is implemented by Relationship.

Mohsen Heydari
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    The distinctions you refer to are artificial. In either case, a "third table" will be created that has identifiers of the two others, plus all attributes for the relationship (which may be zero). – Erwin Smout Sep 08 '13 at 12:14
  • The relationship (0-n) between Entity1 and Entity2 illustrated in the sample will not result the third table. Just the many-to-many relationship will result a new table. Any Association will result a new table. – Mohsen Heydari Sep 09 '13 at 08:25
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Look at it as if you are trying to express something in different languages, or in different "versions" of the same language.

When you want to express "you are an idiot", you could plain say "you are an idiot", or applying the art of typical British understatement, you could say something like "I think there are certain key elements of the problem you seem to have overlooked". Those two sentences are nowhere near the same, but they might very well express the very same intended meaning.

It's just the same with ER diagramming. WHAT you want to express does not have a single unique [graphical in this case] way of expressing it. WHAT you want to express is semantics, HOW you express it is syntax. Same semantics can be expressed with different syntaxes, and that is not a bug, it's a feature !

Erwin Smout
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