Blogonaut

55699: Another question.

55699 talks about some issues with ternary relationships. Reading his post, the key thing I would emphasize is that the ternary relationship represents when three things must be present for an instance of the ternary associative entity type to exist. So, thinking about the ternary relationship between customers, salespeople, and used cars in ER example 3, that relationship is saying that each of the three entities must be present for the transaction to occur.

55699: Lost, early in the game

55699 talks about some issues he is having with ER diagrams. Specifically, he would like some intuitions on why you would want to place the foreign key from the table on the one side of the relationship into the the table on the many side of the relationship when we have a one-to-many relationship.

Here's my intuition as far as I recall it. Basically, if you took the foreign key from the many side of the relationship and placed it in the one side, you would get a repeating group. Think of sections and classrooms. One classroom can hold many sections. Now, if I placed the primary key from section as a foreign key in the classroom table, I would get a repeating group. However, if I place the primary key from classroom into section, I won't get a repeating group because a section can only have one classroom

icemountain: ER Diagram Questions Pt2

icemountain talks about ER diagrams exercise 2.

I should probably rewrite some of the answers to the questions. The way I have been talking about it for the past two renditions of the class, I list all of the entities, even the associative ones. I started doing that last year because it seemed simpler and more consistent to me. The answers on the web do not include the associative entities.

You will construct an ER diagram as part of your first class project.

Big Dog: Ternary Relationships

Big Dog asks about ternary relationships. To clarify some of his issues:

• For an entry to exist in the table underlying the ternary relationship, all three entities must be present.
• Cardinality in the ternary relationship represents the maximal cardinality. Minimal cardinality is always assumed to be one.

jrtrana: Single Hash Marks in ER Models

jrtrana talks about issues interpreting the hash mark for ternary entities.

As explained here, the key point is that the hash mark means different things depending on the entity type. In a ternary relationship, we only represent the maximum cardinality. The minimum cardinality is always assumed to be one.

maulin: Confusion in ER modeling

maulin talks about confusion he is experiencing with sub-entities in ER diagrams. Well, let's hit it point-by-point:

• How can you have a disjoint sub-entity without it being required? Think of it this way. If you choose a to use a sub-entity, you must use one or the other. However, you are not required to use a sub-entity.
• What is the difference between required and disjoint? Required means that you must use a sub-entity. Disjoint means that if you use a sub-entity, you must use one or the other. Something could be disjoint without being required, and vice versa.

This link provides full details. Since I will not be emphasizing this on the test, I will not use a lot of class time on this topic.

club306: WHY BLOG?

club306 asks why people blog. Is it just because of the requirement? Hosup, I refer you to this great article posted by Jon Lee. Also, take a look at the class questions people are posting.

computers: ER Diagrams a little easier!

computers talks about some strategies for analyzing ER diagrams. Way to go, Nick! I like the step-by-step approach.

Icehouse: Remembering Foreign Keys

Icehouse talks about how to know where to insert foreign keys. I like the summary and the initiative to provide an alternative take. The class notes from 9/23 provide an equivalent take.

jrtrana: Accessing Articles

jrtrana talks about not being able to access the articles for 9/28. I handed out hard copies in class because they require a subscription. Would anyone like copies of the articles?