CHAPTER
1
Introduction
Solutions for the Practice Exercises of Chapter 1
Practice Exercises
1.1 This chapter has described several major advantages of a database system. What
are two disadvantages?
Answer:
Two disadvantages associated with database systems are listed below.
a. Setup of the database system requires more knowledge, money, skills, and
time.
b. The complexity of the database may result in poor performance.
1.2 List five ways in which the type declaration system of a language such as Java
or C++ differs from the data definition language used in a database.
Answer:
a. Executing an action in the DDL results in the creation of an object in the
database; in contrast, a programming language type declaration is simply
an abstraction used in the program.
b. Database DDLs allow consistency constraints to be specified, which pro-
gramming language type systems generally do not allow. These include
domain constraints and referential integrity constraints.
c. Database DDLs support authorization, giving different access rights to
different users. Programming language type systems do not provide such
protection (at best, they protect attributes in a class from being accessed
by methods in another class).
d. Programming language type systems are usually much richer than the SQL
type system. Most databases support only basic types such as different
1
2 Chapter 1 Introduction
types of numbers and strings, although some databases do support some
complex types such as arrays and objects.
e. A database DDL is focused on specifying types of attributes of relations;
in contrast, a programming language allows objects and collections of ob-
jects to be created.
1.3 List six major steps that you would take in setting up a database for a particular
enterprise.
Answer:
Six major steps in setting up a database for a particular enterprise are:
• Define the high-level requirements of the enterprise (this step generates a
document known as the system requirements specification.)
• Define a model containing all appropriate types of data and data relation-
ships.
• Define the integrity constraints on the data.
• Define the physical level.
• For each known problem to be solved on a regular basis (e.g., tasks to be
carried out by clerks or web users), define a user interface to carry out the
task, and write the necessary application programs to implement the user
interface.
• Create/initialize the database.
1.4 Suppose you want to build a video site similar to YouTube. Consider each of the
points listed in Section 1.2 as disadvantages of keeping data in a file-processing
system. Discuss the relevance of each of these points to the storage of actual
video data, and to metadata about the video, such as title, the user who uploaded
it, tags, and which users viewed it.
Answer:
• Data redundancy and inconsistency. This would be relevant to metadata to
some extent, although not to the actual video data, which are not updated.
There are very few relationships here, and none of them can lead to redun-
dancy.
• Difficulty in accessing data. If video data are only accessed through a few
predefined interfaces, as is done in video sharing sites today, this will not
be a problem. However, if an organization needs to find video data based
on specific search conditions (beyond simple keyword queries), if metadata
were stored in files it would be hard to find relevant data without writing
application programs. Using a database would be important for the task of
finding data.
Practice Exercises 3
• Data isolation. Since data are not usually updated, but instead newly cre-
ated, data isolation is not a major issue. Even the task of keeping track of
who has viewed what videos is (conceptually) append only, again making
isolation not a major issue. However, if authorization is added, there may
be some issues of concurrent updates to authorization information.
• Integrity problems. It seems unlikely there are significant integrity con-
straints in this application, except for primary keys. If the data are dis-
tributed, there may be issues in enforcing primary key constraints. Integrity
problems are probably not a major issue.
• Atomicity problems. When a video is uploaded, metadata about the video
and the video should be added atomically, otherwise there would be an
inconsistency in the data. An underlying recovery mechanism would be
required to ensure atomicity in the event of failures.
• Concurrent-access anomalies. Since data are not updated, concurrent access
anomalies would be unlikely to occur.
• Security problems. These would be an issue if the system supported autho-
rization.
1.5 Keyword queries used in web search are quite different from database queries.
List key differences between the two, in terms of the way the queries are specified
and in terms of what is the result of a query.
Answer:
Queries used in the web are specified by providing a list of keywords with no spe-
cific syntax. The result is typically an ordered list of URLs, along with snippets
of information about the content of the URLs. In contrast, database queries
have a specific syntax allowing complex queries to be specified. And in the rela-
tional world the result of a query is always a table.
