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enter image description here

The arrows represent the many to one relationships.

From my understanding of the diagram:

Suppliers can only relate to one relationship of car and part. For example Tom's warehouse can only sell a steering wheel to a Ford truck. But Tom's warehouse selling a radio to the Ford truck can't happen due to the constraint.

The Ford truck can only be related to one relationship of a supplier and part. So the Ford truck can only have one supplier, supplying one part to the Ford truck.

How do two many-to-one relationships work inside a ternary ERD diagram?

philipxy
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sam
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  • [Why should I not upload images of code/data/errors when asking a question?](https://meta.stackoverflow.com/q/285551/3404097) An ERD is an image of DDL. – philipxy Jan 18 '23 at 05:05
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    What published DB design textbook/reference are you following? What definitions? What method? What are its steps? Where is the 1st place you are stuck? Why/how are you stuck? Right now you are essentially asking us to (re)write a textbook for an unknown method with bespoke tutorial with no details on what you misunderstand or do or don't understand. [ask] [Help] Basic questions are faqs. [research effort](https://meta.stackoverflow.com/q/261592/3404097) [“help me"](https://meta.stackoverflow.com/q/284236/3404097) [homework](https://meta.stackoverflow.com/q/334822/3404097) – philipxy Jan 18 '23 at 05:06
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    "The arrows represent the many to one relationships." No, they don't. A ternary relationship is not multiple binary relationships & an ERD diamond is one relationship not many. In the ERM "relationship" mean association, not participation/FK. PS Please tell us exactly where you got this & with what else & in what context & textbook, etc. – philipxy Jan 18 '23 at 05:11
  • [Explanation of ER model to functional dependencies solution](https://stackoverflow.com/a/61316402/3404097) [Is this a one to many or many to many relationship?](https://stackoverflow.com/a/41809487/3404097) – philipxy Jan 18 '23 at 06:11

1 Answers1

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In the family of ER modeling notations, your diagram uses Chen's notation. According to Chen himself:

A relationship set is a set of relationships of the same type. (...) A relationship set can also be defined on more than two entity sets. (...) In the entity-relationship diagram, a relationship set is represented by a diamond-shaped box with lines connecting to the related entity sets.

In Chen's notation, the "cardinalities" are indicated explicitly on the edges, with a 1, or a letter for many. According to your question, you assume that the relationship is N:M:1, then it means that:

  • for any given car and any given supplier, there may be only one part,
  • for any given part and any given supplier, there may be many cars,
  • for any given part and any given car, there may be many suppliers.

You can find more explanation in this article, which analysis the different possible cardinalities in a ternary relationship and explains that:

The cardinality constraint of an entity in a ternary relationship is defined by a pair of two entity instances associated with the other single entity instance.

Now, it seems important to underline that the arrow is not used by Chen (except for attributes). I know there are several extended ERD notation, but as far as I know, none associates arrow with multiplicity. So I'm not sure how you come to a N:M:1. Looking at the domain itself, I'd rather suspect a N:M:P.

Christophe
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