An Introduction to Database Systems 8Ed - C J Date - Solutions Manual Episode 1 Part 3 pdf

Copyright (c) 2003 C. J. Date page 3.7

and what the primary and foreign keys are (it isn't so important

to know exactly what the sample data values are!)." Mention the

fact that Fig. 3.8 is repeated inside the back cover of the book,

for ease of subsequent reference.

Answers to Exercises

3.1 As usual, some of the following definitions elaborate slightly

on those given in the body of the chapter.

• The term automatic navigation refers to the fact that (in a

relational system) the process of "navigating" around the

stored data in order to implement user requests is performed

automatically by the system, not manually by the user.

• A base relvar──also known as a real relvar [3.3]──is a relvar

that has independent or autonomous existence. More precisely,

it's a relvar that isn't a derived relvar (q.v.). It's not

necessarily the same thing as a "stored relvar."

• The catalog is a set of system relvars whose purpose is to

contain descriptors regarding the various objects that are of

interest to the system itself, such as base relvars, views,

indexes, users, integrity constraints, security constraints,

and so on.

• The term closure (of relational operations) refers to the

fact that (a) the output from any relational operation is the

same kind of object as the input──they're all relations──and

so (b) the output from one operation can become input to

another. Closure implies that we can write nested (relation￾valued) expressions.

Note: We stress the point that when we say that the output

from each operation is another relation, we are talking from a

conceptual point of view. We don't necessarily mean to imply

that the system actually has to materialize the result of

every individual operation in its entirety. In fact, of

course, the system tries very hard not to, if such

materialization is logically unnecessary (see the brief

discussion of pipelined evaluation in the body of the

chapter).

• Commit is the operation that signals successful end-of￾transaction. Any updates made to the database by the

transaction in question are now "made permanent" and become

visible to other transactions.

Copyright (c) 2003 C. J. Date page 3.8

• A derived relvar is a relvar whose value at any given time is

the result of evaluating a specified relational expression,

typically involving other relvars (ultimately, base relvars).

Note that (like a base relvar) a derived relvar is still a

variable!*──in other words, the term "relvar" does not refer

just to base relvars; moreover, derived relvars must be

updatable (for otherwise they cannot be said to be variables).

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* To be more precise, a derived relvar is a variable if and only

if its defining relational expression involves at least one

relvar; otherwise it would be more accurate to think of it as a

relation constant (a "relcon"?), and it wouldn't be updatable.

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• A foreign key is a column or combination of columns in one

relvar whose values are required to match those of the primary

key in some other relvar (or possibly in the same relvar).

Note: This definition is only approximate. A more precise

definition is given in Chapter 9 (where, among other things,

the point is stressed that a foreign key is a set of columns

and a foreign key value is a set of values──in fact, a

(sub)tuple).

• Join is a relational operation that joins two relations

together on the basis of common values in a common column.

Note: This definition is only approximate. A more precise

definition is given in Chapter 7.

• Optimization is the process of deciding how to implement user

access requests. In other words, it's the process of deciding

how to perform automatic navigation (q.v.).

• A predicate is a truth-valued function. Every relation has a

corresponding predicate that defines (loosely) "what the

relation means." Each row in a given relation denotes a

certain true proposition, obtained from the predicate by

substituting certain argument values of the appropriate type

for the parameters of the predicate ("instantiating the

predicate"). Note: These remarks are all true of relvars as

well as relations, mutatis mutandis.

• The primary key of a given relvar is a column or combination

of columns in that relvar whose values can be used to identify

rows within that relvar uniquely (in other words, it's a