<s>
Data	B-General_Concept
integration	I-General_Concept
involves	O
combining	O
data	O
residing	O
in	O
different	O
sources	O
and	O
providing	O
users	O
with	O
a	O
unified	O
view	O
of	O
them	O
.	O
</s>
<s>
Data	B-General_Concept
integration	I-General_Concept
appears	O
with	O
increasing	O
frequency	O
as	O
the	O
volume	O
(	O
that	O
is	O
,	O
big	B-Application
data	I-Application
)	O
and	O
the	O
need	O
to	O
share	O
existing	O
data	O
explodes	O
.	O
</s>
<s>
Data	B-General_Concept
integration	I-General_Concept
encourages	O
collaboration	O
between	O
internal	O
as	O
well	O
as	O
external	O
users	O
.	O
</s>
<s>
The	O
data	O
being	O
integrated	O
must	O
be	O
received	O
from	O
a	O
heterogeneous	B-General_Concept
database	I-General_Concept
system	I-General_Concept
and	O
transformed	O
to	O
a	O
single	O
coherent	O
data	O
store	O
that	O
provides	O
synchronous	O
data	O
across	O
a	O
network	O
of	O
files	O
for	O
clients	O
.	O
</s>
<s>
A	O
common	O
use	O
of	O
data	B-General_Concept
integration	I-General_Concept
is	O
in	O
data	B-Application
mining	I-Application
when	O
analyzing	O
and	O
extracting	O
information	O
from	O
existing	O
databases	O
that	O
can	O
be	O
useful	O
for	O
Business	B-General_Concept
information	I-General_Concept
.	O
</s>
<s>
Issues	O
with	O
combining	O
heterogeneous	O
data	O
sources	O
are	O
often	O
referred	O
to	O
as	O
information	B-Application
silos	I-Application
,	O
under	O
a	O
single	O
query	O
interface	O
have	O
existed	O
for	O
some	O
time	O
.	O
</s>
<s>
In	O
the	O
early	O
1980s	O
,	O
computer	O
scientists	O
began	O
designing	O
systems	O
for	O
interoperability	O
of	O
heterogeneous	B-General_Concept
databases	I-General_Concept
.	O
</s>
<s>
The	O
first	O
data	B-General_Concept
integration	I-General_Concept
system	O
driven	O
by	O
structured	O
metadata	O
was	O
designed	O
at	O
the	O
University	O
of	O
Minnesota	O
in	O
1991	O
,	O
for	O
the	O
Integrated	O
Public	O
Use	O
Microdata	O
Series	O
(	O
IPUMS	O
)	O
.	O
</s>
<s>
IPUMS	O
used	O
a	O
data	O
warehousing	O
approach	O
,	O
which	O
extracts	O
,	O
transforms	O
,	O
and	O
loads	O
data	O
from	O
heterogeneous	O
sources	O
into	O
a	O
unique	O
view	O
schema	B-Application
so	O
data	O
from	O
different	O
sources	O
become	O
compatible	O
.	O
</s>
<s>
By	O
making	O
thousands	O
of	O
population	O
databases	O
interoperable	O
,	O
IPUMS	O
demonstrated	O
the	O
feasibility	O
of	O
large-scale	O
data	B-General_Concept
integration	I-General_Concept
.	O
</s>
<s>
The	O
data	O
warehouse	O
approach	O
offers	O
a	O
tightly	B-Application
coupled	I-Application
architecture	O
because	O
the	O
data	O
are	O
already	O
physically	O
reconciled	O
in	O
a	O
single	O
queryable	O
repository	O
,	O
so	O
it	O
usually	O
takes	O
little	O
time	O
to	O
resolve	O
queries	B-Language
.	O
</s>
<s>
This	O
problem	O
frequently	O
emerges	O
when	O
integrating	O
several	O
commercial	O
query	O
services	O
like	O
travel	O
or	O
classified	O
advertisement	O
web	B-Application
applications	I-Application
.	O
</s>
<s>
the	O
trend	O
in	O
data	B-General_Concept
integration	I-General_Concept
favored	O
the	O
loose	B-Application
coupling	I-Application
of	O
data	O
and	O
providing	O
a	O
unified	O
query-interface	O
to	O
access	O
real	O
time	O
data	O
over	O
a	O
mediated	O
schema	B-Application
(	O
see	O
Figure	O
2	O
)	O
,	O
which	O
allows	O
information	O
to	O
be	O
retrieved	O
directly	O
from	O
original	O
databases	O
.	O
</s>
<s>
This	O
approach	O
relies	O
on	O
mappings	O
between	O
the	O
mediated	O
schema	B-Application
and	O
the	O
schema	B-Application
of	O
original	O
sources	O
,	O
and	O
translating	O
a	O
query	O
into	O
decomposed	O
queries	B-Language
to	O
match	O
the	O
schema	B-Application
of	O
the	O
original	O
databases	O
.	O
</s>
<s>
Such	O
mappings	O
can	O
be	O
specified	O
in	O
two	O
ways	O
:	O
as	O
a	O
mapping	O
from	O
entities	O
in	O
the	O
mediated	O
schema	B-Application
to	O
entities	O
in	O
the	O
original	O
sources	O
(	O
the	O
"	O
Global-as-View	O
"	O
(	O
GAV	O
)	O
approach	O
)	O
,	O
or	O
as	O
a	O
mapping	O
from	O
entities	O
in	O
the	O
original	O
sources	O
to	O
the	O
mediated	O
schema	B-Application
(	O
the	O
"	O
Local-as-View	O
"	O
(	O
LAV	O
)	O
approach	O
)	O
.	O
</s>
<s>
The	O
latter	O
approach	O
requires	O
more	O
sophisticated	O
inferences	O
to	O
resolve	O
a	O
query	O
on	O
the	O
mediated	O
schema	B-Application
,	O
but	O
makes	O
it	O
easier	O
to	O
add	O
new	O
data	O
sources	O
to	O
a	O
(	O
stable	O
)	O
mediated	O
schema	B-Application
.	O
</s>
<s>
some	O
of	O
the	O
work	O
in	O
data	B-General_Concept
integration	I-General_Concept
research	O
concerns	O
the	O
semantic	O
integration	O
problem	O
.	O
</s>
<s>
A	O
common	O
strategy	O
for	O
the	O
resolution	O
of	O
such	O
problems	O
involves	O
the	O
use	O
of	O
ontologies	O
which	O
explicitly	O
define	O
schema	B-Application
terms	O
and	O
thus	O
help	O
to	O
resolve	O
semantic	O
conflicts	O
.	O
</s>
<s>
This	O
approach	O
represents	O
ontology-based	B-General_Concept
data	I-General_Concept
integration	I-General_Concept
.	O
</s>
<s>
it	O
was	O
determined	O
that	O
current	O
data	B-Application
modeling	I-Application
methods	O
were	O
imparting	O
data	O
isolation	O
into	O
every	O
data	B-General_Concept
architecture	I-General_Concept
in	O
the	O
form	O
of	O
islands	O
of	O
disparate	O
data	O
and	O
information	B-Application
silos	I-Application
.	O
</s>
<s>
This	O
data	O
isolation	O
is	O
an	O
unintended	O
artifact	O
of	O
the	O
data	B-Application
modeling	I-Application
methodology	O
that	O
results	O
in	O
the	O
development	O
of	O
disparate	O
data	O
models	O
.	O
</s>
<s>
One	O
enhanced	O
data	B-Application
modeling	I-Application
method	O
recasts	O
data	O
models	O
by	O
augmenting	O
them	O
with	O
structural	O
metadata	O
in	O
the	O
form	O
of	O
standardized	O
data	O
entities	O
.	O
</s>
<s>
Since	O
2011	O
,	O
data	B-Application
hub	I-Application
approaches	O
have	O
been	O
of	O
greater	O
interest	O
than	O
fully	O
structured	O
(	O
typically	O
relational	O
)	O
Enterprise	O
Data	O
Warehouses	O
.	O
</s>
<s>
Since	O
2013	O
,	O
data	B-General_Concept
lake	I-General_Concept
approaches	O
have	O
risen	O
to	O
the	O
level	O
of	O
Data	B-Application
Hubs	I-Application
.	O
</s>
<s>
These	O
approaches	O
combine	O
unstructured	O
or	O
varied	O
data	O
into	O
one	O
location	O
,	O
but	O
do	O
not	O
necessarily	O
require	O
an	O
(	O
often	O
complex	O
)	O
master	O
relational	O
schema	B-Application
to	O
structure	O
and	O
define	O
all	O
data	O
in	O
the	O
Hub	O
.	O
</s>
<s>
Data	B-General_Concept
integration	I-General_Concept
plays	O
a	O
big	O
role	O
in	O
business	O
regarding	O
data	O
collection	O
used	O
for	O
studying	O
the	O
market	O
.	O
</s>
<s>
Organizations	O
are	O
more	O
frequently	O
using	O
data	B-Application
mining	I-Application
for	O
collecting	O
information	O
and	O
patterns	O
from	O
their	O
databases	O
,	O
and	O
this	O
process	O
helps	O
them	O
develop	O
new	O
business	O
strategies	O
to	O
increase	O
business	O
performance	O
and	O
perform	O
economic	O
analyses	O
more	O
efficiently	O
.	O
</s>
<s>
Compiling	O
the	O
large	O
amount	O
of	O
data	O
they	O
collect	O
to	O
be	O
stored	O
in	O
their	O
system	O
is	O
a	O
form	O
of	O
data	B-General_Concept
integration	I-General_Concept
adapted	O
for	O
Business	B-General_Concept
intelligence	I-General_Concept
to	O
improve	O
their	O
chances	O
of	O
success	O
.	O
</s>
<s>
Consider	O
a	O
web	B-Application
application	I-Application
where	O
a	O
user	O
can	O
query	O
a	O
variety	O
of	O
information	O
about	O
cities	O
(	O
such	O
as	O
crime	O
statistics	O
,	O
weather	O
,	O
hotels	O
,	O
demographics	O
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
Traditionally	O
,	O
the	O
information	O
must	O
be	O
stored	O
in	O
a	O
single	O
database	O
with	O
a	O
single	O
schema	B-Application
.	O
</s>
<s>
A	O
data-integration	O
solution	O
may	O
address	O
this	O
problem	O
by	O
considering	O
these	O
external	O
resources	O
as	O
materialized	B-Application
views	I-Application
over	O
a	O
virtual	B-General_Concept
mediated	I-General_Concept
schema	I-General_Concept
,	O
resulting	O
in	O
"	O
virtual	O
data	B-General_Concept
integration	I-General_Concept
"	O
.	O
</s>
<s>
This	O
means	O
application-developers	O
construct	O
a	O
virtual	O
schema	B-Application
—	O
the	O
mediated	O
schema	B-Application
—	O
to	O
best	O
model	O
the	O
kinds	O
of	O
answers	O
their	O
users	O
want	O
.	O
</s>
<s>
These	O
adapters	O
simply	O
transform	O
the	O
local	O
query	O
results	O
(	O
those	O
returned	O
by	O
the	O
respective	O
websites	O
or	O
databases	O
)	O
into	O
an	O
easily	O
processed	O
form	O
for	O
the	O
data	B-General_Concept
integration	I-General_Concept
solution	O
(	O
see	O
figure	O
2	O
)	O
.	O
</s>
<s>
When	O
an	O
application-user	O
queries	B-Language
the	O
mediated	O
schema	B-Application
,	O
the	O
data-integration	O
solution	O
transforms	O
this	O
query	O
into	O
appropriate	O
queries	B-Language
over	O
the	O
respective	O
data	O
sources	O
.	O
</s>
<s>
Finally	O
,	O
the	O
virtual	B-General_Concept
database	I-General_Concept
combines	O
the	O
results	O
of	O
these	O
queries	B-Language
into	O
the	O
answer	O
to	O
the	O
user	O
's	O
query	O
.	O
</s>
<s>
The	O
virtual	O
ETL	O
solutions	O
leverage	O
virtual	B-General_Concept
mediated	I-General_Concept
schema	I-General_Concept
to	O
implement	O
data	O
harmonization	O
;	O
whereby	O
the	O
data	O
are	O
copied	O
from	O
the	O
designated	O
"	O
master	O
"	O
source	O
to	O
the	O
defined	O
targets	O
,	O
field	O
by	O
field	O
.	O
</s>
<s>
Advanced	O
data	B-General_Concept
virtualization	I-General_Concept
is	O
also	O
built	O
on	O
the	O
concept	O
of	O
object-oriented	O
modeling	O
in	O
order	O
to	O
construct	O
virtual	B-General_Concept
mediated	I-General_Concept
schema	I-General_Concept
or	O
virtual	O
metadata	O
repository	O
,	O
using	O
hub	O
and	O
spoke	O
architecture	O
.	O
</s>
<s>
Therefore	O
,	O
data	B-General_Concept
virtualization	I-General_Concept
as	O
well	O
as	O
data	O
federation	O
depends	O
upon	O
accidental	O
data	O
commonality	O
to	O
support	O
combining	O
data	O
and	O
information	O
from	O
disparate	O
data	O
sets	O
.	O
</s>
<s>
The	O
theory	O
of	O
data	B-General_Concept
integration	I-General_Concept
forms	O
a	O
subset	O
of	O
database	O
theory	O
and	O
formalizes	O
the	O
underlying	O
concepts	O
of	O
the	O
problem	O
in	O
first-order	O
logic	O
.	O
</s>
<s>
Applying	O
the	O
theories	O
gives	O
indications	O
as	O
to	O
the	O
feasibility	O
and	O
difficulty	O
of	O
data	B-General_Concept
integration	I-General_Concept
.	O
</s>
<s>
Connections	O
to	O
particular	O
databases	O
systems	O
such	O
as	O
Oracle	O
or	O
DB2	O
are	O
provided	O
by	O
implementation-level	O
technologies	O
such	O
as	O
JDBC	B-Language
and	O
are	O
not	O
studied	O
at	O
the	O
theoretical	O
level	O
.	O
</s>
<s>
Data	B-General_Concept
integration	I-General_Concept
systems	O
are	O
formally	O
defined	O
as	O
a	O
tuple	O
where	O
is	O
the	O
global	O
(	O
or	O
mediated	O
)	O
schema	B-Application
,	O
is	O
the	O
heterogeneous	O
set	O
of	O
source	O
schemas	O
,	O
and	O
is	O
the	O
mapping	O
that	O
maps	O
queries	B-Language
between	O
the	O
source	O
and	O
the	O
global	O
schemas	O
.	O
</s>
<s>
Both	O
and	O
are	O
expressed	O
in	O
languages	O
over	O
alphabets	O
composed	O
of	O
symbols	O
for	O
each	O
of	O
their	O
respective	O
relations	B-Application
.	O
</s>
<s>
The	O
mapping	O
consists	O
of	O
assertions	O
between	O
queries	B-Language
over	O
and	O
queries	B-Language
over	O
.	O
</s>
<s>
When	O
users	O
pose	O
queries	B-Language
over	O
the	O
data	B-General_Concept
integration	I-General_Concept
system	O
,	O
they	O
pose	O
queries	B-Language
over	O
and	O
the	O
mapping	O
then	O
asserts	O
connections	O
between	O
the	O
elements	O
in	O
the	O
global	O
schema	B-Application
and	O
the	O
source	O
schemas	O
.	O
</s>
<s>
A	O
database	O
over	O
a	O
schema	B-Application
is	O
defined	O
as	O
a	O
set	O
of	O
sets	O
,	O
one	O
for	O
each	O
relation	O
(	O
in	O
a	O
relational	B-Application
database	I-Application
)	O
.	O
</s>
<s>
The	O
database	O
corresponding	O
to	O
the	O
source	O
schema	B-Application
would	O
comprise	O
the	O
set	O
of	O
sets	O
of	O
tuples	O
for	O
each	O
of	O
the	O
heterogeneous	O
data	O
sources	O
and	O
is	O
called	O
the	O
source	O
database	O
.	O
</s>
<s>
The	O
database	O
corresponding	O
to	O
the	O
virtual	B-General_Concept
mediated	I-General_Concept
schema	I-General_Concept
is	O
called	O
the	O
global	O
database	O
.	O
</s>
<s>
GAV	O
systems	O
model	O
the	O
global	O
database	O
as	O
a	O
set	O
of	O
views	B-Language
over	O
.	O
</s>
<s>
Query	B-Language
processing	I-Language
becomes	O
a	O
straightforward	O
operation	O
due	O
to	O
the	O
well-defined	O
associations	O
between	O
and	O
.	O
</s>
<s>
The	O
burden	O
of	O
complexity	O
falls	O
on	O
implementing	O
mediator	O
code	O
instructing	O
the	O
data	B-General_Concept
integration	I-General_Concept
system	O
exactly	O
how	O
to	O
retrieve	O
elements	O
from	O
the	O
source	O
databases	O
.	O
</s>
<s>
In	O
a	O
GAV	O
approach	O
to	O
the	O
example	O
data	B-General_Concept
integration	I-General_Concept
system	O
above	O
,	O
the	O
system	O
designer	O
would	O
first	O
develop	O
mediators	O
for	O
each	O
of	O
the	O
city	O
information	O
sources	O
and	O
then	O
design	O
the	O
global	O
schema	B-Application
around	O
these	O
mediators	O
.	O
</s>
<s>
The	O
designer	O
would	O
likely	O
then	O
add	O
a	O
corresponding	O
element	O
for	O
weather	O
to	O
the	O
global	O
schema	B-Application
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
in	O
LAV	O
,	O
the	O
source	O
database	O
is	O
modeled	O
as	O
a	O
set	O
of	O
views	B-Language
over	O
.	O
</s>
<s>
The	O
benefit	O
of	O
an	O
LAV	O
modeling	O
is	O
that	O
new	O
sources	O
can	O
be	O
added	O
with	O
far	O
less	O
work	O
than	O
in	O
a	O
GAV	O
system	O
,	O
thus	O
the	O
LAV	O
approach	O
should	O
be	O
favored	O
in	O
cases	O
where	O
the	O
mediated	O
schema	B-Application
is	O
less	O
stable	O
or	O
likely	O
to	O
change	O
.	O
</s>
<s>
In	O
an	O
LAV	O
approach	O
to	O
the	O
example	O
data	B-General_Concept
integration	I-General_Concept
system	O
above	O
,	O
the	O
system	O
designer	O
designs	O
the	O
global	O
schema	B-Application
first	O
and	O
then	O
simply	O
inputs	O
the	O
schemas	O
of	O
the	O
respective	O
city	O
information	O
sources	O
.	O
</s>
<s>
The	O
designer	O
would	O
add	O
corresponding	O
elements	O
for	O
weather	O
to	O
the	O
global	O
schema	B-Application
only	O
if	O
none	O
existed	O
already	O
.	O
</s>
<s>
Then	O
programmers	O
write	O
an	O
adapter	O
or	O
wrapper	O
for	O
the	O
website	O
and	O
add	O
a	O
schema	B-Application
description	O
of	O
the	O
website	O
's	O
results	O
to	O
the	O
source	O
schemas	O
.	O
</s>
<s>
The	O
theory	O
of	O
query	B-Language
processing	I-Language
in	O
data	B-General_Concept
integration	I-General_Concept
systems	O
is	O
commonly	O
expressed	O
using	O
conjunctive	O
queries	B-Language
and	O
Datalog	B-Language
,	O
a	O
purely	O
declarative	O
logic	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
One	O
can	O
loosely	O
think	O
of	O
a	O
conjunctive	O
query	O
as	O
a	O
logical	O
function	O
applied	O
to	O
the	O
relations	B-Application
of	O
a	O
database	O
such	O
as	O
"	O
where	O
"	O
.	O
</s>
<s>
While	O
formal	O
languages	O
like	O
Datalog	B-Language
express	O
these	O
queries	B-Language
concisely	O
and	O
without	O
ambiguity	O
,	O
common	O
SQL	B-Language
queries	B-Language
count	O
as	O
conjunctive	O
queries	B-Language
as	O
well	O
.	O
</s>
<s>
In	O
terms	O
of	O
data	B-General_Concept
integration	I-General_Concept
,	O
"	O
query	O
containment	O
"	O
represents	O
an	O
important	O
property	O
of	O
conjunctive	O
queries	B-Language
.	O
</s>
<s>
The	O
two	O
queries	B-Language
are	O
said	O
to	O
be	O
equivalent	O
if	O
the	O
resulting	O
sets	O
are	O
equal	O
for	O
any	O
database	O
.	O
</s>
<s>
This	O
is	O
important	O
because	O
in	O
both	O
GAV	O
and	O
LAV	O
systems	O
,	O
a	O
user	O
poses	O
conjunctive	O
queries	B-Language
over	O
a	O
virtual	O
schema	B-Application
represented	O
by	O
a	O
set	O
of	O
views	B-Language
,	O
or	O
"	O
materialized	O
"	O
conjunctive	O
queries	B-Language
.	O
</s>
<s>
Integration	O
seeks	O
to	O
rewrite	O
the	O
queries	B-Language
represented	O
by	O
the	O
views	B-Language
to	O
make	O
their	O
results	O
equivalent	O
or	O
maximally	O
contained	O
by	O
our	O
user	O
's	O
query	O
.	O
</s>
<s>
This	O
corresponds	O
to	O
the	O
problem	O
of	O
answering	O
queries	B-Language
using	O
views	B-Language
(	O
AQUV	O
)	O
.	O
</s>
<s>
Each	O
element	O
in	O
the	O
user	O
's	O
query	O
corresponds	O
to	O
a	O
substitution	O
rule	O
just	O
as	O
each	O
element	O
in	O
the	O
global	O
schema	B-Application
corresponds	O
to	O
a	O
query	O
over	O
the	O
source	O
.	O
</s>
<s>
Query	B-Language
processing	I-Language
simply	O
expands	O
the	O
subgoals	O
of	O
the	O
user	O
's	O
query	O
according	O
to	O
the	O
rule	O
specified	O
in	O
the	O
mediator	O
and	O
thus	O
the	O
resulting	O
query	O
is	O
likely	O
to	O
be	O
equivalent	O
.	O
</s>
<s>
In	O
LAV	O
systems	O
,	O
queries	B-Language
undergo	O
a	O
more	O
radical	O
process	O
of	O
rewriting	O
because	O
no	O
mediator	O
exists	O
to	O
align	O
the	O
user	O
's	O
query	O
with	O
a	O
simple	O
expansion	O
strategy	O
.	O
</s>
<s>
The	O
integration	O
system	O
must	O
execute	O
a	O
search	O
over	O
the	O
space	O
of	O
possible	O
queries	B-Language
in	O
order	O
to	O
find	O
the	O
best	O
rewrite	O
.	O
</s>
<s>
the	O
GQR	O
algorithm	O
is	O
the	O
leading	O
query	O
rewriting	O
algorithm	O
for	O
LAV	O
data	B-General_Concept
integration	I-General_Concept
systems	O
.	O
</s>
<s>
This	O
type	O
of	O
data	B-General_Concept
integration	I-General_Concept
is	O
especially	O
challenging	O
for	O
ecological	O
and	O
environmental	O
data	O
because	O
metadata	O
standards	O
are	O
not	O
agreed	O
upon	O
and	O
there	O
are	O
many	O
different	O
data	O
types	O
produced	O
in	O
these	O
fields	O
.	O
</s>
<s>
National	O
Science	O
Foundation	O
initiatives	O
such	O
as	O
Datanet	O
are	O
intended	O
to	O
make	O
data	B-General_Concept
integration	I-General_Concept
easier	O
for	O
scientists	O
by	O
providing	O
cyberinfrastructure	O
and	O
setting	O
standards	O
.	O
</s>
<s>
The	O
Research	O
Data	O
Alliance	O
,	O
has	O
more	O
recently	O
explored	O
creating	O
global	O
data	B-General_Concept
integration	I-General_Concept
frameworks	O
.	O
</s>
