[Thursday, Jan 27, 10:55]

> I have a question regarding your handout 5-9 and 5-11.
>
> A)  In 5-9, the triangle, "has," and the rectangle, "Position," are connected
by the double line because of the minimal cardinality of 1.  Is this the same as
 saying that it is so because of the total participation?
>
> B)  In 5-11, the entity, "Bill," and the relationship, "for," are also connect
ed with the cardinality of (1,1), which I think means the total participation.
Why is this connecting line a single one?  Wouldn't it be more appropriate if it
's double?

Thank you for your email.
I think I was a bit confusing in this point yesterday.
There are two notations for ER-Diagrams (well, really more than ten ...):
- "My" notation, which uses (min,max) cardinalities and no double lines
  for expressing cardinalities (only for weak entities, you can see this as
  an underlined line).
  It is correct that minimum cardinality 1 is the same as saying that the
  entity has total participation in the relationship.
- The "Elmasri/Navathe" notation, which uses 1:M, M:M, and 1:1 for maximum
  cardinalities and double lines for minimum cardinality 1 (total participation)
  and single lines for minimum cardinality 0 (single participation).
What I wanted to say was that in the Elmasri/Navathe notation, the use of the
double line for weak entities and minimum cardinality 1 is consistent, since
a weak entity implies minimum cardinality 1.
I guess that was not helpful since I otherwise didn't talk very much about the
Elmasri/Navathe notion.
In "my" notation, the double line is needed to mark clearly who is parent and
who is child in the relationship. The parent could be itself a weak entity,
and then it would be at least difficult to see which entity inherits the key.
(this is actually a problem in the Elmasri/Navathe notation when the parent
entity has the minimum cardinality 1 in the relationship with the weak entity.)
So think of the double line as an underlined line, just as key attributes are
underlined.

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[Friday, Jan 28, 11:45]

> I find an error in your slide #5-33.  In the slide, Optional Attributes
> vs. Specialization, "Faculty Member" entity is a superclass while "Faculty
> Member with Phone" entity is its subclass.  Therefore, the arrow should
> point to "Faculty Member", representing "Faculty Member with Phone" is-a
> "Faculty Member".  However, the arrow in the slide points to opposit
> direction.

yes, you are right. Congratulations.

Last term I used the triangle in the converse direction, and followed the
notation developed in Braunschweig where I studied.
But now UML has become so important, and UML uses the triangle in this
direction, so I wanted to change it
(actually, this was a suggestion by a student).
But obviously I forgot this slide.

You earn 1% of extra credit for telling me about this mistake
(because this is an important error, typing errors earn a bit less).