Re: ER Diagrams

[dialist <pvspam-dialist hacklab net>]

On Tue, 2003-04-08 at 18:24, Carol Farlow Lerche wrote:
> Executive summary
> of this post: it would be useful if dia/tedia2sql supported the
> variants of ER notation known as crow's foot and IDEF1X as well as UML
> notation,

Perhaps you don't understand the problem we Dia-using DBAs have
completely.

Crow's foot is simply a graphical representation of a concept,
specifically a 1-x relationship. The circle or line is just to denote
optionality, which is a way of defining primary keys on the tables.

Dia presently does not have any shapeset that does real Database Design.
The ER shapeset is not really for database design so much as true Entity
Relationship design, to which database design is only theoretically
similar. The only shape set Dia supports presently that is close to what
DBAs need is UML. That doesn't mean we DBAs are HAPPY about it, just
that it's the only tool we have presently.

tedia2sql only parses UML presently because UML is the only database
modelling shape set that is sufficient in Dia right now. It's been a
long-time task on the "to-do list" of tedia2sql developers to modify Dia
itself to have a database design shapeset that includes crow's feet (and
I assume this IDEF1X you mention).

Again, I urge you strongly to look at the tedia2sql Dia usage
documentation at http://tedia2sql.tigris.org/usingtedia2sql.html and use
UML until we can get a nice Database Modelling shape set built.

> because conceptual data models in these notations have been
> found by some to be easier to convey to the business stakeholders than
> ones using UML class notation.

It shouldn't be too difficult to understand. The UML association has an
"aggregate" end which is similar-looking to the crow's feet. I have
built UML-based ERDs and my developers and managers understood fairly
quickly that the UML ----<> relationship is equivalent to the crow's
foot ----<- relationship (sorry for my ASCII art... best I can do on
short notice).

Right now UML doesn't have an optionality concept, so tedia2sql only
parses a 1-0,1 or 1-many relationship. It doesn't generate primary keys,
either. As you can tell, we're not 100% the way to database design, but
it's good enough for most projects at this point.

> There are two notation
> variants that are used in "real life": crow's foot and IDEF1X.  Tools
> like "ER Studio" support both.  The difference is really in the
> connectors used to join entities and represent optionality and
> cardinality. The following url is a reference that shows the crow's
> foot notation:

I think the DBAs on the list understand the visual representation fine.
It's been a problem to find someone that:

1. Cares enough to create a new shape set for Dia,
2. Has the technical ability to do so,
3. Has the time to do so.

You obviously have (1) and possibly (2), but then, so most of the DBAs
that subscribe to this list. We really need someone that also has (3),
and that's where we've fallen short thus far.

> I would not say that UML has eliminated the use of ER diagrams, as was
> stated in a prior post, although there is certainly a faction that
> holds that UML diagramming is the only thing worth using.

Ah. You misunderstood what he was saying. He is saying no DBAs use the
ER diagrams, but instead use a class representation *LIKE* UML to do
diagramming. He is saying exactly what you are saying, but he did not
appear to know what "crow's feet" are, which is simply a visual
representation of the "class related to another class in a 1-x
relationship."

> This is a religious war.

No! Just a slight misunderstanding. Don't get too upset just yet. We
really do need your help!

-- 
Tim Ellis
Senior Database Architect and author, tedia2sql (http://tedia2sql.tigris.org)
If this helped you, http://svcs.affero.net/rm.php?r=philovivero