What is the difference between MER (Relationship Entity Model) and DER (Relationship Entity Diagram)?

Question

What is the difference between an MER diagram and an ERI diagram, with examples.

Answer 1

The MER is what you want to do effectively, is the tool to create data models and their relationships, in general that will then be produced in a relational database. The ERI is just a visual way to assemble and analyze this model.

The MER is abstract, it is only a concept, we can say that it exists only in thought, although you can put it on paper in a disorganized way. The ERI is something with its own rules of how to design this model that will represent the data organization that it will use in the database, so we can say that it is a little more concrete, to be visible and transferable between people and even tools in communicating what will be (or has been) done. Of course, he’s still an abstraction.

You can only present the ERI, only it is visual. The MER can explain, but the moment you set an example in an organized and visual way is making an DER. If you’ve seen a diagram already have an example.

There are several different notations for a DER. One of them is from Wikipedia:

Answer 2

The MER (Entity-Relationship Model) is a language that aims to describe conceptual database models. It was proposed by Peter Chen in 1976. Through it it is possible to represent concepts such as entities and relationships. It follows part of the notation (or alphabet) of the MER language.

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Just as there are several languages (or languages) in the world (such as Portuguese or Japanese), we know that, although different (alphabet, syntax, semantics, etc.), all present the same objective of allowing communication between people.

Therefore, just as there is the MER language for describing conceptual models, there are other languages that allow to achieve the same goal.

After the proposal of Peter Chen (there in 1976), there were other proposals for languages with different alphabets (or notations). A very famous one is James Martin’s (also known as the chicken foot notation), exemplified below. In this case, the main difference between them would be the alphabet, keeping the MER concepts (entities, relationships, attributes, etc).

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It is through modeling languages that we can create the Entity-Relationship Diagrams (conceptual models) of our database projects. The following are examples of Entity-Relationship Diagrams in each notation (Peter Chen and James Martin, respectively):

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In short:

1. MER: language that allows describing conceptual database models data in Peter Chen’s notation;
2. DER: model created through a modeling language, which uses the concepts present in the MER (as entity and relationship), and may use different notations.