Databases Reference
In-Depth Information
abstraction), in the schema simplification approach, the user may build a proper view of the original
schema which cannot be extracted by the schema itself at any of its levels of abstraction.
Query formulation
is the fundamental activity in the process of data retrieval. The query
strategy by schema navigation has the characteristic of concentrating on a concept (or a group of
concepts) and moving from it in order to reach other concepts of interest, on which further query
predicates may be specified. Such a strategy differs according to the type of path followed during
the navigation (see Figure
2.6
for an example of unconnected path).
BRIDGE
CANCEL
OK
PERSON
STATE
SURNAME
S_NAME
1
NAME
FISCAL_C
DRIV_LIC
DATE_BIR
S_INHABI
MAN
STATE
Figure 2.6:
Unconnected path in QBD* (from
Angelaccio et al.
,
1990
).
A second strategy for query formulation is
by subqueries
. In this case, the query is formulated
by composing partial results. The third strategy for query formulation is
by matching
. This is based
on the idea of presenting the structure of a possible answer that is matched against the stored data.
The last strategy for query formulation is
by range selection
, allowing a search condi-
tioned by a given range on multi-key data sets to be performed. The query is formulated through
direct manipulation of graphical widgets, such as buttons, sliders, and scrollable lists, with one
widget being used for every key. An interesting implementation of such a technique is
dynamic
query
(
Ahlberg and Shneiderman
,
1994
). The user can either indicate a range of numerical values
(with a range slider) or a sequence of names alphabetically ordered (with an alpha slider). Given
a query, a new query is easily formulated by moving the position of a slider with a mouse: this is
supposed to give a sense of power but also of fun to the user, who is challenged to try other queries
and see how the result is modified. Usually, input and output data are of the same type and may even
coincide. Overall, visual interaction is based on the visual perception capabilities of the user.
