What is the difference between MER (relationship entity model) and DER (relationship entity diagram)?
What is the difference between a MER and a DER diagram, with examples.
2 answers
MER is what you want to do effectively, it is the tool for creating data models and their relationships, in general that will then be produced in a relational database. DER 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 only exists in thought, although you can put it on paper in a disorganized way. DER is something with its own rules of how to draw this model that will represent the organization of data that will use in the database, so we can say that it is a little more concrete, not least because it is visible and transferable between people and even tools in the communication of what will be (or was) done. Of course, he is still an abstraction.
Can only present the DER, only it is visual. The MER can explain, but at the moment it is assembling an example in an organized and visual way it is doing a DER. If you've seen a diagram, you already have an example.
There are several notations different for a DER. One of them picking up from Wikipedia:
MER (Entity-Relationship Model) is a language that has the purpose of describing 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.
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 have the same objective of allowing communication between people.
Thus, in the same way that there is the MER language for the description of conceptual models, there are other languages that allow achieving the same goal.
After the proposal of Peter Chen (there in 1976), other language proposals with different alphabets (or notations) appeared. A very famous one is that of James Martin (also known as the foot notation of chicken), exemplified below. In the case, the main difference between them would be the alphabet, keeping the concepts of the MER (entities, relationships, attributes, etc.).
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):
Summarizing:
- MER: language that allows you to describe conceptual bench models of data in the notation of Peter Chen;
- DER: model created by means of a modeling language, which uses the concepts present in the MER (as entity and relationship), and can use different notations.