Database Reference
In-Depth Information
Table 4. Excerpt of the EmpDB relation
Id
Salary
Age
…
1
45000
62
2
38750
24
3
37500
28
where
p
1
,
p
2
are fuzzy predicates.
Table 6 shows the fuzzy relation corresponding to the query 'Age =
young
AND Salary=
reasonable
'.
Note that the
min
and
max
operators may be replaced by any
t-norm
and
t-conorm
operators (Klement,
Mesiar & Pap, 2000) to model the conjunction and disjunction connectives, respectively.
FQUERY
In (Zadrozny & Kacprzyk, 1996), the authors proposed FQUERY, an extension of the Microsoft Ac-
cess SQL language with capability to manipulate fuzzy terms. More specifically, they proposed to take
into account the following types of fuzzy terms: fuzzy values, fuzzy comparison operators, and fuzzy
quantifiers. Given a query involving fuzzy terms, the matching degree of relevant answers is calculated
according to the semantics of fuzzy terms (Zadeh, 1999). In addition to the syntax and semantics of the
extended SQL, the authors have also proposed a scheme for the elicitation and manipulation of fuzzy
terms to be used in queries. FQUERY has been one of the first implementations demonstrating the use-
fulness of fuzzy querying features for a traditional database.
SQLf
In contrast to both Tahani's approach and FQUERY which concentrated on the fuzzification of conditions
appearing in the 'WHERE' clause of the SQL's SELECT statement, the query language SQLf (Bosc &
Pivert, 1992; Bosc & Pivert, 1995; Bosc & Pivert, 1997) allows the introduction of fuzzy terms into SQL
wherever they make sense. Indeed, all the operations of the relational algebra (implicitly or explicitly
used in SQL's SELECT instruction) are redefined in such a way that the equivalences that occur in the
crisp SQL are preserved. Thus, the projection, selection, join, union, intersection, Cartesian product and
Table 5. Fuzzy relations
(
a
)
EmpDB
Age=
young
Id
Salary
Age
…
γ
Age=
young
1
45000
62
0.0
2
38750
24
0.5
3
37500
28
0.25
(
b
) E
mpDB
Salary=
reasonable
Id
Salary
Age
…
γ
Salary=
reasonable
1
45000
62
1
2
38750
24
0.75
3
37500
28
0.25
