Note: Reader mail (rm) posts are my exchanges with readers that raise fundamental issues. I may improve language for clarity and amplify with notes for the benefit of readers.
I've written more than once about Chen's E/RM (see references) and recently a reader emailed me a reaction to one of these writings, offering E/RM as evidence that properties were not necessary in conceptual modeling -- everything could be expressed with just entities and relationships. That is, of course, incorrect, but I was working at the time on my latest series of posts on Relationships and the RDM, which was going to address that issue too and I intended to refer him to it. But knowing that David McGoveran knew Chen in the 80s, I asked him if he had any comments. He thought that something broader and more forceful regarding Chen's work was in order and suggested some text. We had some discussion on the subject and decided that I will post my reply (ON PROPERTIES IN CONCEPTUAL MODELING) and publish his reply to the reader later, which I do below.
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I knew Chen back in the 1980s (at least well enough to correspond with him and have several telephone conversations and was one of the early users of his ER Diagrammer tool.) At that time I was using the tool for diagrams with more entities than any other user. I only met him in person much later at a DCI conference.
Note that Chen used “data [model]" in a broader, less rigidly (and less rigorously) defined way than did Codd. At the time Chen introduced E/RM, “network" was widely considered a data model even though it was impossible to separate logical and physical levels. Indeed, Chen shows the network "model" as having no logical level in his 1976 paper [1]. The “Great Debate" between Codd and Bachman had taken place in 1974, but the consequences had not permeated the industry. Chen attempted to provide an independent and encompassing approach without being involved in the arguments and promoted E/RM as having wide use.
The claim that the ER diagrams of Chen's E/RM did not use properties (which he called attributes) for conceptual modeling is false.
Chen enumerated] four levels of ERD modeling and maps between them: conceptual, “information structure”, logical, and physical. He described how those levels appeared in each of four distinct “data models”: E-R, network, relational, and entity-set.
Chen defined relationships as being between entities, with the assumption that entities were defined in such a way that there were no relationships among the attributes of an entity. That is, entities at the conceptual level (Chen's level 1) have to have a certain simplicity if relationships between properties are to be ignored.
E/RM, in moving from conceptual level to the logical level (level 2) maps names of entities and their attributes (a.k.a. properties!) to data. If properties do not exist at the conceptual level, what is the source of these attributes? How do they become data, having a possible set of values?
Chen explicitly cited use of normalization of conceptual entities to 3NF at the logical level for relational. If properties are not involved, this correspondence becomes vacuous (useless). He thus acknowledged and used dependency theory, which is about relationships among attributes!
Chen created and sold in the 1980s a PC-based tool called ER-Diagrammer. It let users create a conceptual ER diagram, and then automatically generated a 3NF logical ERD. (As I recall, it also could then generate SQL to create the physical - at least for Oracle.) This demonstrates that Chen’s definitions allowed the conceptual definition of an entity to be incomplete with respect to relationships captured in the conceptual model and that properties were essential.
I suggest everyone actually read Chen's Turing Award paper. Anecdotal accounts of what E/RM is and how it should be used can be misleading and dangerous.
The only reasonable conclusion is that Chen (the inventor of E/RM) understood both the importance and necessity of properties at the conceptual level. If E/RM is being used to model the conceptual level without properties (Chen's attributes), it is obviously being misused!
Ed. Note
Yes, of course Chen incorporated properties, how could it be otherwise, given that:
“... singular names, property terms and class terms correspond to common sense concepts of individual beings, traits or activities of individual beings and sets of beings who share some common traits or activities ... This three-fold distinction is an important part of our Western intellectual heritage and is deeply embedded not only in our thought habits, but also in our language.” --R. Olson, MEANING AND ARGUMENT: ELEMENTS OF LOGIC
Emphasis mine.
References
Chen, P. P., The Entity-Relationship Model-Toward a Unified View of Data, ACM Transactions on Database Systems, Vol. 1, No. 1. March 1976. (See especially Fig.1 on page 3)
Understanding Conceptual vs.Data Modeling Part 2 E/RM Models Reality, RDM Models Data
Understanding Conceptual vs. Data Modeling Part 1 Data Model - The RDM Is, the E/RM Isn't
Understanding Data Modeling Part 2 E/RM and RDM
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