Databases Reference
In-Depth Information
c.
Every subset of a tuple is a tuple.
2.4 The term
domain
is usually found in texts on relational theory, but it wasn't mentioned in the body of the
chapter. What do you make of this fact?
2.5
Define the terms
proposition
and
predicate
. Give examples.
2.6
State the predicates for relvars S, P, and SP from the suppliers-and-parts database.
2.7 Let
DB
be any database you happen to be familiar with and let
R
be any relvar in
DB
. What's the predicate
for
R?
Note:
The point of this exercise is to get you to apply some of the ideas discussed in the body of this chapter
to your own data, in an attempt to get you thinking about data in general in such terms. Obviously the exercise has
no unique right answer.
2.8 Explain
The Closed World Assumption
in your own terms. Could there be such a thing as
The Open World
Assumption?
2.9
Give definitions, as precise as you can make them, of the terms
tuple
and
relation
.
2.10
State as precisely as you can what it means for (a) two tuples to be equal; (b) two relations to be equal.
2.11 A tuple is a set (a set of components); so do you think it might make sense to define versions of the usual set
operators (union, intersection, etc.) that apply to tuples?
2.12 To repeat, a tuple is a set of components. But the empty set is a legitimate set; thus, we could define an
empty tuple
to be a tuple where the pertinent set of components is empty. What are the implications? Can you think
of any uses for such a tuple?
2.13 A key is a set of attributes and the empty set is a legitimate set; thus, we could define an
empty key
to be a
key where the pertinent set of attributes is empty. What are the implications? Can you think of any uses for such a
key?
2.14 A predicate has a set of parameters and the empty set is a legitimate set; thus, a predicate could have an
empty set of parameters. What are the implications?
2.15 The normalization discipline makes heavy use of the relational operators projection and join. Give
definitions, as precise as you can make them, of these two operators. Also, have a go at defining the attribute
renaming operator (RENAME in
Tutorial D
).
2.16
The operators of the relational algebra form a closed system. What do you understand by this remark?
