SPECIAL IMPLEMENTATION CONSIDERATIONS - Database Design and Development: An Essential Guide for IT Professionals - page 383

Queries on ORDER relation

Print all order numbers for Customer “Robert Smith”

OrdrNo OrdrDate OrdrAmt CustName

P. Robert Smith

Find the order numbers and order dates of all orders with amounts more than $750

OrdrNo OrdrDate OrdrAmt CustName

P. P. > 750

Find the order numbers and order amounts of all orders not for “Bill Jones”

OrdrNo

OrdrDate OrdrAmt CustName

P. P. NOT Bill Jones

Figure 13-6 QBE queries.

Query-by-Example Query-by-example (QBE) adopts a visual approach to
implementing the data model through templates. You enter an example value into
the template for data retrieval. Queries are expressed as examples that are provided
through the language.
QBE was developed in early 1970s at IBM's Watson Research Center. Later, the
QBE data manipulation language component was used in IBM's Query Manage-
ment Facility (QMF). Today, many relational DBMSs including Microsoft ACCESS
and IBM's Paradox use QBE in one form or another.
In QBE, you use a skeletal two-dimensional table to express your query.
Queries are in the form of tables, not free-form text with statements or com-
mands. You fill in a skeletal table with example rows. An example row consists of
example data elements and constants. The result of the query is also presented as a
table.
Figure 13-6 shows examples of QBE queries.
QBE has provisions to perform the following:

• Indicate conditions for data retrieval through the use of condition boxes
• Display results taking values from several database relations
• Order the display of result rows
• Perform aggregations
• Data maintenance—deletion, insertion, and update

Quel Developed at the University of California, Berkeley, Quel was introduced
as the implementation language for the Ingres database system. Three basic types
of clauses exist: range of, retrieve, and where. The underlying structure resembles
that of relational calculus.
You can express most of the queries just by using these three standard clauses.
The first clause, “range of” declares a tuple or row variable taking on values of tuples