Comments on: What is entity-relationship model?

By: Quassnoi

Quassnoi — Mon, 01 Mar 2010 08:17:47 +0000

@Goncalo: no, just missed :)

By: Gonçalo

Gonçalo — Mon, 01 Mar 2010 00:10:47 +0000

Very, very good article.

Just a note: do the arrows between the attributes in the marriage diagrams represent any concept, or have you just missed those?

By: Quassnoi

Quassnoi — Tue, 29 Dec 2009 13:30:33 +0000

@openfrog: MS Visio.

By: openfrog

openfrog — Tue, 29 Dec 2009 13:27:44 +0000

Thanks a lot for this great article. The first time I really feel I have understood what an ER-Diagram really is all about and how it works. I came here through StackOverflow.com.

May I ask, what tool did you use to create those diagrams?

By: Quassnoi

Quassnoi — Sat, 05 Dec 2009 13:08:30 +0000

@Praveen: with ER, you should define entities, relationships and attributes. The diagram in the link you provided shows the relational design which provides the tables layout (so that we can generate a database schema) but gives too few hints on what will be stored in the tables. What is Long_Fields on diagram above? Is it an entity or a relationship or an offloaded attribute? You cannot tell from the name of the fields and the table design.

By: Praveen

Praveen — Sat, 05 Dec 2009 05:35:04 +0000

Hi, I’m using the following format to design ER, after all it looks much alike the Table Structure design.

http://www.meds-sdmm.dfo-mpo.gc.ca/meds/Prog_Nat/NCIS/Definition/Ncis_ER_diagram.gif

(found this on google)

I’m wondering what’s the difference between ER and Table structure.
Thanks

By: uberVU - social comments

uberVU - social comments — Sun, 22 Nov 2009 08:43:01 +0000

Social comments and analytics for this post…

This post was mentioned on Twitter by HybridDBA: Nice ERD overview / review: http://bit.ly/Voj8Q...

By: Quassnoi

Quassnoi — Tue, 20 Oct 2009 09:50:41 +0000

@rexem: Diamonds for a relationship are used in Chen’s notation. This allows relationships to have their own attributes and relationships (like, say, a marriage date in the example above).

However, a relationship still needs to be uniquely defined by the entities related, which distinguishes it from an entity.

By: rexem

rexem — Tue, 20 Oct 2009 05:15:21 +0000

Coming from Oracle Designer – what you’ve referred to as the “ER model”, I’ve known as the “Logical ERD” while the “relational database [model]” was called the “Physical ERD”. The difference isn’t immediately obvious on simple models, but if you compare the attributes/columns, foreign key(s) won’t show be in the attribute list – they would be in the physical model because the resulting table was generated. Another thing on the Logical model was that optional columns would be noted. Logical models would also show arcs, and the term escapes me but tables encapsulated by tables to show attribute inheritance…

I haven’t seen the diagram format you mention, using diamonds – only rectangles and crowsfeet. The rules to crowsfeet were that they pointed either down or to the East (right). The lines were dotted to indicate optional relationships, and the crowsfeet attached to a table to show a many relationship – crowsfeet on either end would mean “many-to-many”. Also, lines shouldn’t cross if they don’t have to.

The diagram feature in SQL Server Management Studio is an example of a physical model. Likewise for the no longer supported Visio Modeller app.