Database Reference
In-Depth Information
Query-by-Example (QBE) is an attractive implementation of domain calculus by
Microsoft Access. The user simply completes a table on the screen in order to make a
query request. Further discussion of domain calculus is omitted from this course. Suffice
it to say that domain calculus has its advantages in more complicated comparisons and
is particularly useful when one is willing to concentrate on domain variables rather then
tuple variables.
8.7 Summary and Concluding Remarks
It's now time to summarize what we have covered in this chapter:
•
Relational calculus is said to be descriptive, meaning, you
describe precisely the activity required. The chapter concentrated
on tuple calculus, i.e., calculus related to the manipulation of
tuple variables.
•
There are a number of standard relational calculus notations
that allow you to succinctly describe data to be retrieved from
relational tables. Retrieval statements essentially involve
manipulating tuple variables using well-formed formulas (WFFs).
A WFF is essentially a simple or complex condition.
•
WFFs often involve the use of free and bound variables. The rules
for these must be strictly followed.
•
Query optimization is the process of obtaining the most efficient
relational algebra equivalent for a relational calculus statement. In
so doing, the basic rules for query optimization must be observed.
•
Domain oriented calculus is calculus related to the manipulation
of domain variables. It is an alternative to tuple calculus.
The language QUEL very closely resembled the relational calculus. The more
contemporary SQL represents exhibits both relational calculus and relational algebra
features.
8.8 Review Questions
1.
Why is relational calculus important? Explain why relational
algebra is said to be prescriptive, and relational calculus is
said to be descriptive. Provide an example to illustrate.
2.
Describe the salient components of relational calculus.
3.
Using the college database described in this chapter, practice
writing relational calculus statements that will yield certain
desirable results.
4.
Explain the concept and process of query optimization. Use
an appropriate example to illustrate.
