BIT320 Remix — ERmodel
November 13, 2005
Exam Question 15
Topics: ERmodel Opinionslug Test
Both Aga and Supriya are correct about Exam Question 15. Since a course can have many sections, C must be correct. Therefore, if you put C, bring your exam to my secretary in E2420 before Thanksgiving to get your points.
In Blogonautic Solutions, 11/13/2005 | Original | Archive | Post to del.icio.us | Technorati
Exam Question 13
Supriya gets right to the heart of the matter when she states:
Since Pink Footsie started the discussion on the test questions, I thought this would be a good time to clarify some of my doubts from the test as well. Question 13 on the test reads:
Based on Diagram 1, whats the minimum number of class meetings a student may participate in?
The correct answer is A-0.
However, I am not sure how to derive this answer from reading the diagram. Now, it is given that a SSC will have many students. Since the cardinality of a student in a SSC is missing, do we read it as unknown, and then assume that a student can be in 0 SSCs for the minimum?
As we stated many times in class, when there is a ternary relationship between entities, entities are not obliged to participate in that ternary relationship. Therefore, the minimum number of times an entity may be represented in such a relationship is 0.
In Blogonautic Solutions, 11/13/2005 | Original | Archive | Post to del.icio.us | Technorati
Question 13 on Exam
Topics: ERmodel
Since Pink Footsie started the discussion on the test questions, I thought this would be a good time to clarify some of my doubts from the test as well. Question 13 on the test reads:
Based on Diagram 1, whats the minimum number of class meetings a student may participate in?The correct answer is A-0.
However, I am not sure how to derive this answer from reading the diagram. Now, it is given that a SSC will have many students. Since the cardinality of a student in a SSC is missing, do we read it as unknown, and then assume that a student can be in 0 SSCs for the minimum? Similarly a class meeting is given to have many SSCs. But the cardinality for SSCs in a class meeting is not given. Does that again mean that a SSC may have 0 class meetings? Hence, combining the two observations, a student may attend 0 class meetings? I am not sure if this is how we are suppose to read it?
However, another way I looked at it was that for every SSC to exist, there should be atleast one student entry, one section and one course. Only then will we have an entry in the SSC table. Hence, a student has to be in one SSC atleast. Now is this intepretation wrong? aNy clarification on this question will be appreciated 
In supriya, 11/13/2005 | Original | Archive | Post to del.icio.us | Technorati
Another view point - Exam Question # 9
Topics: ERmodel
Pink Footsie gives a good insight into question 9 on the test as she states:
Based on the diagram many of us could argue that there was no date, therefore under just interpreting the diagram it would be answer A. The reason that is not true is because the question is a little tricky. Specifically the words: and the project description imply the need to apply both the diagram and the description to your reasoning.
However, I looked at the question in a different way. I used reverse engineering to say that the relationship b/w class meeting and course section is not M:M for sure, since a cardinality of 1 is indicated on the course section side. Hence, this relationship is either 1:1 or 1:M. So, the CourseSectionID should be included as a foreign key (if 1:M) and not a primary key in the classMeeting table. Hence, roomID+professorID+courseSectionID is not a candidate primary key for the classMeeting table since it can be further broken down into subsets and we do not need the courseSectionID in the primary key. However, I think this combination of fields can be a super key. Is that right?
Does this reasoning make sense or is it too abstract? Any comments?
In supriya, 11/13/2005 | Original | Archive | Post to del.icio.us | Technorati
Contesting Question 15 - II
Topics: ERmodel Opinionslug
I like Pink Footsie also marked C-Many as the answer for Question 15. As she states:
The answer given is B. “1″ but I would like to say that the C. “Many” interpretation is true. If you read the diagram from the side of class meeting then it’s true there can only be one section meeting at that time but if you read it from the course side (course can have many sections and therefore sections can have many meetings) the maximum relationship based on interpretation ends up being many, which is the side the question is asking about. Any takers? Looking at the diagram might help clarify what I’m trying to say.
I had a similar thought process where a course can have many sections. Since the cardinality on the class meeting side is unknown, it can be assumed as one or many. Assuming this cardinality to be many, I said that a course section can have many class meetings. But another confusing point was that roomID, ProfessorID and courseSectionID are the primary keys for classMeeting, which made me think that there can be only one entry for a course section for one course. However, we can still have these 3 attributes in the primary key, and have the same courseSectionID with different roomID/ProfessorID in the class meeting table and not violate the primary key constraint. With these assumptions, I think that the answer is Many.
I understand that what I typed is quite confusing but I couldn’t figure out a better way to explain my reasoning. Feel free to ask questions or pls explain why you think the answer is B-1 and not C-Many.
In supriya, 11/13/2005 | Original | Archive | Post to del.icio.us | Technorati
November 10, 2005
Contesting Test question 15 on ERD
Question 15 reads:
Still on diagram 2, assuming the primary key of class meeting is roomID, professor ID, and coursesectionID, what is the maximum number of class meetings a course may have?
The answer given is B. “1″ but I would like to say that the C. “Many” interpretation is true. If you read the diagram from the side of class meeting then it’s true there can only be one section meeting at that time but if you read it from the course side (course can have many sections and therefore sections can have many meetings) the maximum relationship based on interpretation ends up being many, which is the side the question is asking about. Any takers? Looking at the diagram might help clarify what I’m trying to say.
In Pink Footsie, 11/10/2005 | Original | Archive | Post to del.icio.us | Technorati
Exam clarification #1: ER portion question 9
I know several people might have had a question regarding number 9 in the ER portion of the exam. Here’s how it reads:
9) Based on Diagram 2 and the project description, a candidate primary key for the ternary relationship Class Meeting is roomID, professorID, and course sectionID.
A. True B. False Correct Answer: B. False.
Based on the diagram many of us could argue that there was no date, therefore under just interpreting the diagram it would be answer A. The reason that is not true is because the question is a little tricky. Specifically the words: and the project description imply the need to apply both the diagram and the description to your reasoning. Sorry for everyone who missed that one.
In Pink Footsie, 11/10/2005 | Original | Archive | Post to del.icio.us | Technorati
October 18, 2005
Counting all those composite entities
Topics: ERmodel classquestions
To answer Kevin’s question:
On the upcoming test, if a question asks us to list or count all of the entities, should we include composite entities?
You should include all entities, including all the composite entities. I found the answer by looking at exercise two and its answer where Bud writes:
Neglecting the associative entities: customer, sales person, model, car, used, new. There are three associative entities: test drive, Sales person, and sold to us. People frequently do not include associative entities. However, I find it easier to just include them based on the “anything in a box is an entity rule”. Answers for the test should include the associative entities.
I hope this helps.
In Pink Footsie, 10/18/2005 | Original | Archive | Post to del.icio.us | Technorati
October 17, 2005
Entities
Topics: ERD ERmodel classquestions
Kevin, in reference to ER Exercise 1, makes the following observation:
On the upcoming test, if a question asks us to list or count all of the entities, should we include composite entities? ER exercise one seems to indicate that we should not. If someone could provide some insight, that would be great.
My simple rule is, “If it is in a box, it is counted as an entity.” The diagram in ER 1 does not put the associative (composite) entities in boxes, so they would not count.
Not all of the exercises may be consistent in application of my rule, but it will be on the test.
In Blogonautic Solutions, 10/17/2005 | Original | Archive | Post to del.icio.us | Technorati
October 16, 2005
[no title]
Topics: ERD ERmodel classquestions
Kevin, in reference to ER Exercise 1, makes the following observation:
On the upcoming test, if a question asks us to list or count all of the entities, should we include composite entities? ER exercise one seems to indicate that we should not. If someone could provide some insight, that would be great.
My simple rule is, “If it is in a box, it is counted as an entity.” The diagram in ER 1 does not put the associative (composite) entities in boxes, so they would not count.
Not all of the exercises may be consistent in application of my rule, but it will be on the test.
In Blogonautic Solutions, 10/16/2005 | Original | Archive | Post to del.icio.us | Technorati
October 9, 2005
Entity vs. Entity Type
Topics: ERmodel classquestions
In response to Aga’s question on Entity versus Entity type,
Realize that this is a different distinction than that between entity types and entities. In this example, BIT320 is one specific instance of the entity type Course and section 2 of BIT320 in Winter 1962 is one specific instance of the entity type Section. Thus, both are entities and neither one is an entity type.”
I think of an entity type as a concept which is not tangible - for example student. So the specific entity for that type will be a particular student such as Melissa or Aditi. Similarly, thinking about the above example, Course is a broader concept which will be the entity type while BIT320 which is a particular course will be its entity.
I am not sure if I can articulate this well, but I think of the broader class as the entity type and then individuals/components of that entity type as entities.
Hope that helps!
In supriya, 10/09/2005 | Original | Archive | Post to del.icio.us | Technorati
September 30, 2005
Data Flow Diagram Software
Topics: ERmodel
Use SmartDraw to design perfect ERD
In del.icio.us/tigerlily23, 09/30/2005 | Original | Archive
September 24, 2005
Entities and Entity Types
Topics: ERmodel classquestions
JB supplies this response to Tigerlily’s question:
For example, if you own a company, you have customers. If you put all of your information about customers in a table, it is called a CUSTOMER entity type. Let’s say your customers include Smith, Jones and Brown. SMITH is an entity because it is an instance of the entity type CUSTOMER.
I like this answer. Jenny clarified what I said in class and put it well. I also appreciate that Tigerlily asked the question in the first place. Often, one talks in class thinking all is clear. It’s good interactions like this that help keep things really getting across.
In Blogonautic Solutions, 09/24/2005 | Original | Archive | Post to del.icio.us | Technorati
September 23, 2005
Entities and Entity Types
Topics: ERmodel classquestions
In response to this question.
As I understand it, an entity type is a table, or a collection of entities. An entity is an instance of an entity type.
For example, if you own a company, you have customers. If you put all of your information about customers in a table, it is called a CUSTOMER entity type. Let’s say your customers include Smith, Jones and Brown. SMITH is an entity because it is an instance of the entity type CUSTOMER.
So yes, an entity is a specific name. It is not just the name of a field.
In jb's blog, 09/23/2005 | Original | Archive | Post to del.icio.us | Technorati
Relationships as Tables?
Topics: ERmodel classquestions
Another question that I had after looking through my notes again is how do we know when a relationship is its own separate table? I think I am confused about why it wouldn’t when “1-mandatory is not on either side (for 1-M or 1-1 relationship).”
In Tigerlily's Blog, 09/23/2005 | Original | Archive | Post to del.icio.us | Technorati
