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I am working with an ERD. It is supposedly a logical model and I am to make a physical model from it. I should be formatting in UML and our DBMS is PostgreSQL.

Some of my research (http://www.1keydata.com/datawarehousing/data-modeling-levels.html // http://en.wikipedia.org/wiki/Logical_data_model#Conceptual.2C_Logical_.26_Physical_Data_Model) indicates that this ERD may have too much information in it to be a logical model and that it may actually be closer to the physical.

My questions are as follows:

  1. What do the bold labels mean?
  2. What do the white "N"s and red "U"s at the end of some entries mean?
  3. What is the difference between a dashed line (relationship) and a solid one?
  4. What is the difference between the "crows foot" and the broken line on either end of the relationship?
  5. Is this closer to the physical model or logical model? What would I have to do to convert it from one to the other?

This is the ERD:

An ERD

niton
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Captain Stack
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  • This is really a relational model, not an ER model, even though the diagram is called an "Entity Relationship Diagram". – Walter Mitty Apr 21 '15 at 11:20
  • @WalterMitty, care to tell what is the difference between a "relational model" and ER diagram? – NoChance Jul 25 '18 at 01:48
  • I am thinking of the ER model as originally developed by Chen. The first part of the wikipedia article describes it pretty well. An ER model is not biased towards a relational impem3ntation. A relational model, on the other hand is biased towards a relational implementation, or perhaps an SQL implementation. https://en.m.wikipedia.org/wiki/Entity–relationship_model – Walter Mitty Jul 27 '18 at 00:49
  • One major difference is that foreign keys are not present in an ER model. Foreign keys, whether or not they are declared, are at the heart of a relational model. – Walter Mitty Jul 27 '18 at 00:51

1 Answers1

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  1. Could bold text indicate primary key attributes?
  2. That's not part of any standard ER modelling notation. By no means certain but my guess would be U means unique, N means nullable.
  3. A solid line means an identifying relationship. Dashed line means a non-identifying one. It's usually not an especially important distinction but look those terms up if you want to know more.
  4. One to many relationship. The crows foot represents the "many" side of the relationship; the short line across is the "one" side. Where the "one" symbol appears at both ends that's a one-to-one relationship.
  5. In the context of information modelling a logical model means a semantic model - a model that's more about the business domain than about an actual database design. Exactly what goes into the logical model and at what level of detail depends a lot on the intended audience for the model and on how you want to use it. Turning it into a "physical" model means making it into a design for a database with the technical features and any changes you would need for your chosen DBMS platform (specific data types for example).

Logical/physical models in the information modelling sense should not be confused with what are termed the logical level and physical level in DBMS architecture and database theory. In principle relational database tables (AKA relation variables) are always "logical" level constructs but in data modelling terms they are part of a so-called "physical" model. That unfortunate choice of modelling terminology is responsible for a lot of confusion and misunderstandings.

nvogel
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  • I thought the same thing about the bold text, but I would have thought the icons on the left would be a better indication of that, with the yellow "keys" being primary keys and the table with a green arrow being foreign keys. That leads me to wonder what the bold is. Is there another more standard way of representing unique and nullable? I'm pretty sure this is ultimately more of a physical model, as logical models don't usually contain datatypes. That said, I think there's a lot of work to be done on both. Thanks for the response! – Captain Stack May 24 '13 at 22:04
  • There are various different notations for ER models, with different ways of showing primary keys and nullability - although alternate keys are quite often not shown at all. – nvogel May 24 '13 at 22:53