DATABASE MANAGEMENT
SYSTEMS part 2
11. Discuss the
alternative terminology that is used in the relational model.
Relations are also
called tables, and sometimes by the older data processing term files. A row is
known as a tuple in the relational model, but may also be referred to as a
record. Finally, relational model attributes are known as table columns and
sometimes as fields.
12. Why are
functional dependencies not equations?
Equations deal with
numerical relationships. A functional dependency deals with the existence
of a determinant relationship
between attributes, regardless of whether or not there is a
numerical
relationship between them. Thus, if we know that there is no hot water every
Wednesday,
No-Hot-Water is functionally dependent on Wednesday. So, if we know it is
Wednesday, then we
know we will have No-Hot-Water. This is a functional dependency, but
not an equation.
13. What is a
foreign key, and what is it used for?
A foreign key is
used to establish relationships among relations in the relational model.
Technically, a
foreign key is a column (or columns) appearing in one relation that is (are)
the
primary key of
another table. Although there may be exceptions, the values in the foreign key
columns usually must
correspond to values existing in the set of primary key values. This
correspondence
requirement is created in a database using a referential integrity constraint
on
the foreign key.
14. What are
insertion and deletion anomalies?
A deletion anomaly
occurs when, by deleting the facts about one entity, we inadvertently
delete facts about
another entity; with one deletion, we lose facts about two entities. For
example, if we
delete the tuple for Student 001289 from a table, we may lose not only the fact
that Student 001289
is in Pierce Hall, but also the fact that he has $200 left in his security
deposit. An
insertion anomaly happens when we encounter the restriction that we cannot
insert a fact about
one entity until we have an additional fact about another entity. For
example, we want to
store the fact that the security deposit for Pierce Hall is $300, but we
cannot enter this
data into the Student relation until a student registers for Pierce Hall.
15. What does it
mean when we say that a relation is in Boyce-Codd Normal Form
(BCNF)?
A relation is in
BCNF when every determinant in the relation is a candidate key. This means
that any possible
primary key can determine all other attributes in the relation. Attributes may
not be determined by
non-candidate key attributes or part of a composite candidate key. Thus
it is said "I
swear to construct my tables so that all nonkey columns are dependent on the
key,
the whole key and
nothing but the key, so help me Codd!"
16. You have been
given a set of tables with data and asked to create a new
database to store
them. When you examine the data values in the tables, what are
you looking for?
(1) Multivalued
dependencies, (2) Functional dependencies, (3) Candidate keys, (4) Primary
keys and (5) Foreign
keys.
17. Why do
normalized tables require more complex SQL when SQL statements are
used in application
programs?
Tables that are
normalized contain data that has been distributed among the tables, but which
may need to be
recombined to answer queries from an application. To recombine the data, the
programmer will have
to use subqueries and/or joins. These SQL structures are more complex
to write than a
simple SELECT statement.
18. What is the
multivalue, multicolumn problem? Include an example not used in
the text.
The multivalue,
multicolumn problem occurs when a table is designed to include multiple
columns that hold
variations of one type of attribute data. One example is where boat owners
have the names of
their boats stored as BOAT_01, BOAT_02 and BOAT_03.
19. Why is the
multivalue, multicolumn problem another form of the multivalued
dependency problem?
Both problems try to
store multiple values on an attribute in a table. In the multivalue,
multiplecolumn
problem, the multiple values are stored in different columns. In the
mutlivalued
dependency problem the multiple values are stored in different rows. In both
cases, the solution
is the same: store the multiple values in a separate table.
20. What is the
inconsistent values problem? Include an example not used in the
text.
The inconsistent
values problem occurs when different users or data sources use slightly
different forms of
the same data value. One example is where automobiles are specified as
"Ford, 2-door,
Red" in one cell and "Red Ford 2-door" in another.
Great and Useful Article.
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