Database Reference
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then in force of (1) (Definition 13) we take
ω
1
(
x
):= ω
2
(
x
)
.
Then we say that the
missing value
of
x
for the valuation ω
1
is discovered
as ω
2
(
x
).
The following algorithm generates an XQuery program for a given schema and a set of TP-XFDs
over this schema. The program discovers all possible missing values, with respect to the given set of
TP-XFDs. We say that in this way the instance is being
repaired
.
Algorithm 2
.
(
generation of XQuery program discovering missing values
)
Input
: A schema
S
(
x
) = /
top
[
E
] and a set
F
of text-valued TP-XFDs over
S
(
x
).
Output
: Program in XQuery over instances of
S
(
x
) returning a repaired version
of the given instance of
S
(
x
).
Method:
xfdToXQuery
(/
top
[
E
]) - identical to the translation function
mappingToXQuery
(
∧
x
(/
top
[
E
]
∧
/
top
[
E
]) in Algorithm 1, except that the rule
(4) is replaced with the rule:
4'. ρ(
l = x
) = <
l
>{
if
($
x
= “
null
”)
then
F
(
x'
)[text() != “
null
”]
/text
()
else
$
x
}
</
l
>,
where (
F
(
x'
),
x
)
∧
F
.
Example 7.
Discovering missing values in an instance of
S
1
(Figure 3) can be done using the XQuery
program (Query2) generated for the schema
S
1
where TP-XFD constraints are:
F
1
(
x
1
):= /
pubs
/
pub
[
title
=
x
1
]/
year
,
and
F
2
(
x
3
):=/
pubs
/
pub
/
author
[
name = x
3
]/
university
.
The corresponding XQuery program is similar to this for Query 1. However, expressions defining
elements
year
and
university
attempt to discover missing values, when the current values of $x2 or $x3
are null:
Query 2:
<pubs>{
for $_v in doc(“I1.xml”)/pubs,
$_v1 in if ($_v[pub]) then $_v/pub else /,
$x1 in if ($_v1[title]) then $_v1/title/text()
else “null”,
$x2 in if ($_v1[year]) then $_v1/year/text()
