Short Answers
- 2.
What is a useful technique for identifying entities in a written description of a
data modeling problem?
One useful approach to identifying entities is to underline all nouns in the problem
description. Most nouns are possible entities. Select an entity that seems central
to the problem and describe it. Then move to the next one.
- 4.
When is a data model well formed and when is it high fidelity?
A data model is well formed when all attributes and entities have meaningful and
unique names. Entities, identifiers, and relationships should be meaningful to the
client.
A high-fidelity data model faithfully describes the real world. All relationships are
recorded and are of the correct degree. There are no compromises and all exceptions
have been taken into account.
A well-formed data model is syntactically correct and a high-fidelity data model is
complete, understandable, and accurate.
- 8.
Why might a data model contract?
A data model might shrink when you generalize structures. Some entities might
actually all be part of a more general entity and thus could be combined into one
entity.
- 10.
What is the difference between a synonym and a homonym?
Synonyms: Different words are used to describe the same concept.
Homonyms: The same word is used for different concepts.
Data Modeling
- 2.
A university has many subject areas (e.g., MIS, Romance languages). Professors
teach in only one subject area, but the same subject area can have many professors.
Professors can teach many different courses in their subject area. An offering of a
course (e.g., Data Management 457, French 101) is taught by only one professor at a
particular time.
- 4.
Steve operates a cinema chain and has given you the following information:
“I have many cinemas. Each cinema can have multiple theaters. Movies are shown
throughout the day starting at 11 a.m. and finishing at 1 a.m. Each movie is given a
two-hour time slot. We never show a movie in more than one theater at a time, but we do
shift movies among theaters because seating capacity varies. I am interested in knowing
how many people, classified by adults and children, attended each showing of a movie. I
vary ticket prices by movie and time slot. For instance, Lassie Get Lost at 11 a.m. is
50 cents for everyone but is 75 cents at 11 p.m.”
- 6.
A famous Greek shipping magnate, Stell, owns many container ships. Containers are
collected at one port and delivered to another port. Customers pay a negotiated fee for
the delivery of each container. Each ship has a sailing schedule that lists the ports
the ship will visit over the next six months. The schedule shows the expected arrival
and departure dates. The daily charge for use of each port is also recorded.
- 8.
A telephone company offers a 10 percent discount to any customer who phones
another person who is also a customer of the company. To be eligible for the discount,
the pairing of the two phone numbers must be registered with the telephone company.
Furthermore, for billing purposes, the company records both phone numbers, start time,
end time, and date of call.
- 10.
A real estate investment company owns many shopping malls. Each mall contains many
shops. To encourage rental of its shops, the company gives a negotiated discount to
retailers who have shops in more than one mall. Each shop generates an income stream
that can vary from month to month because rental is based on a flat rental charge and a
negotiated percentage of sales revenue. Also, each shop has monthly expenses for
scheduled and unscheduled maintenance. The company uses the data to compute its monthly
net income per square meter for each shop and for ad hoc querying.
- 12.
Draw a data model for the following freight table taken from a mail order catalog.
Merchandise
subtotals |
Regular delivery
7–10 days |
Rush delivery
4–5 business days |
Express delivery
1–2 business days |
| Up to $30.00 |
$4.95 |
$9.95 |
$12.45 |
| $30.01–$65.00 |
$6.95 |
$11.95 |
$15.45 |
| $65.01–$125.00 |
$8.95 |
$13.95 |
$20.45 |
| $125.01+ |
$9.95 |
$15.95 |
$25.45 |
