Data Integration
Zachary G. Ives
University of Pennsylvania
CIS 550 – Database & Information Systems
November 14, 2007
Challenges of Mapping Schemas
In a perfect world, it would be easy to match up items from one
schema with another
 Every table would have a similar table in the other schema
 Every attribute would have an identical attribute in the other schema
 Every value would clearly map to a value in the other schema
Real world: as with human languages, things don’t map clearly!




May have different numbers of tables – different decompositions
Metadata in one relation may be data in another
Values may not exactly correspond
It may be unclear whether a value is the same
2
Different Aspects to Mapping
Schema matching / ontology alignment
How do we find correspondences between attributes?
Entity matching / deduplication / record linking / etc.
How do we know when two records refer to the same
thing?
Mapping definition
 How do we specify the constraints or transformations
that let us reason about when to create an entry in one
schema, given an entry in another schema?
Let’s see one influential approach to schema matching…
3
The LSD (Learning Source
Descriptions) System
Suppose user wants to integrate 100 data sources
1. User:


manually creates mappings for a few sources, say 3
shows LSD these mappings
2. LSD learns from the mappings


“Multi-strategy” learning incorporates many types of info
in a general way
Knowledge of constraints further helps
3. LSD proposes mappings for remaining 97 sources
4
Example
address
location
Mediated schema
price
agent-phone
listed-price
phone
description
comments
Schema of realestate.com
location
listed-price
phone
comments
realestate.com Miami, FL $250,000 (305) 729 0831 Fantastic house
Boston, MA $110,000 (617) 253 1429 Great location
...
...
...
...
homes.com
Learned hypotheses
If “phone” occurs
in the name =>
agent-phone
If “fantastic” &
“great”
occur frequently
in data values =>
description
price
contact-phone
extra-info
$550,000 (278) 345 7215 Beautiful yard
$320,000 (617) 335 2315 Great beach
...
...
...
5
LSD’s Multi-Strategy Learning
Use a set of base learners
 Each exploits well certain types of information:
 Name learner looks at words in the attribute names
 Naïve Bayes learner looks at patterns in the data values
 Etc.
Match schema elements of a new source
 Apply the base learners
 Each returns a score
 For different attributes one learner is more useful than another
 Combine their predictions using a meta-learner
Meta-learner
 Uses training sources to measure base learner accuracy
 Weighs each learner based on its accuracy
6
Training the Learners
Mediated schema
address
location
price
agent-phone
listed-price
phone
description
comments
Schema of realestate.com
Name Learner
realestate.com
<location> Miami, FL </>
<listed-price> $250,000</>
<phone> (305) 729 0831</>
<comments> Fantastic house </>
<location> Boston, MA </>
<listed-price> $110,000</>
<phone> (617) 253 1429</>
<comments> Great location </>
(location, address)
(listed-price, price)
(phone, agent-phone)
(comments, description)
...
Naive Bayes Learner
(“Miami, FL”, address)
(“$ 250,000”, price)
(“(305) 729 0831”, agent-phone)
(“Fantastic house”, description)
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...
Applying the Learners
Mediated schema
Schema of homes.com
area
day-phone extra-info
<area>Seattle, WA</>
<area>Kent, WA</>
<area>Austin, TX</>
address
Name Learner
Naive Bayes
Name Learner
Naive Bayes
<day-phone>(278) 345 7215</>
<day-phone>(617) 335 2315</>
<day-phone>(512) 427 1115</>
<extra-info>Beautiful yard</>
<extra-info>Great beach</>
<extra-info>Close to Seattle</>
price
agent-phone
Meta-Learner
Meta-Learner
description
(address,0.8), (description,0.2)
(address,0.6), (description,0.4)
(address,0.7), (description,0.3)
(address,0.7), (description,0.3)
(agent-phone,0.9), (description,0.1)
(address,0.6), (description,0.4)
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Putting It All Together: LSD System
Training Phase
Matching Phase
Mediated schema
Source schemas
Data listings
Training data
for base learners
L1
L2
Lk
Domain
Constraints
User Feedback
Constraint Handler
Mapping Combination
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Mappings between Schemas
LSD provides attribute correspondences, but not complete
mappings
Mappings generally are posed as views: define relations in one
schema (typically either the mediated schema or the source
schema), given data in the other schema
 This allows us to “restructure” or “recompose + decompose” our
data in a new way
We can also define mappings between values in a view
 We use an intermediate table defining correspondences – a
“concordance table”
 It can be filled in using some type of code, and corrected by hand
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A Few Mapping Examples
 Movie(Title, Year, Director,
 PieceOfArt(ID, Artist, Subject,
Editor, Star1, Star2)
Title, TypeOfArt)
PieceOfArt(I, A, S, T, “Movie”) :- Movie(T, Y, A, _, S1, S2),
ID = T || Y, S = S1 || S2
 Movie(Title, Year, Director,
Editor, Star1, Star2)
 MotionPicture(ID, Title, Year)
Participant(ID, Name, Role)
Movie(T, Y, D, E, S1, S2) :- MotionPicture(I, T, Y),
Participant(I, D, “Dir”), Participant(I, E, “Editor”),
Participant(I, S1, “Star1”), Participant(I, S2, “Star2”)
T1
CustID CustName
1234
Smith, J.
T2 PennID EmpName
46732
John Smith
Need a concordance table from CustIDs to PennIDs
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Two Important Approaches
 TSIMMIS [Garcia-Molina+97] – Stanford
 Focus: semistructured data (OEM), OQL-based language (Lorel)
 Creates a mediated schema as a view over the sources
 Spawned a UCSD project called MIX, which led to a company now
owned by BEA Systems
 Other important systems of this vein: Kleisli/K2 @ Penn
 Information Manifold [Levy+96] – AT&T Research
 Focus: local-as-view mappings, relational model
 Sources defined as views over mediated schema
 Requires a special
 Led to peer-to-peer integration approaches (Piazza, etc.)
 Focus: Web-based queriable sources
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TSIMMIS
 One of the first systems to support semi-structured
data, which predated XML by several years: “OEM”
 An instance of a “global-as-view” mediation system
 We define our global schema as views over the sources
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XML vs. Object Exchange Model
<book>
<author>Bernstein</author>
<author>Newcomer</author>
<title>Principles of TP</title>
</book>
<book>
<author>Chamberlin</author>
<title>DB2 UDB</title>
O1: book {
</book>
O2: author { Bernstein }
O3: author { Newcomer }
O4: title { Principles of TP }
}
O5: book {
O6: author { Chamberlin }
O7: title { DB2 UDB }
}
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Queries in TSIMMIS
Specified in OQL-style language called Lorel
 OQL was an object-oriented query language that looks like SQL
 Lorel is, in many ways, a predecessor to XQuery
Based on path expressions over OEM structures:
select book
where book.title = “DB2 UDB” and book.author = “Chamberlin”
This is basically like XQuery, which we’ll use in place of Lorel
and the MSL template language. Previous query restated =
for $b in AllData()/book
where $b/title/text() = “DB2 UDB” and
$b/author/text() = “Chamberlin”
return $b
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Query Answering in TSIMMIS
Basically, it’s view unfolding, i.e., composing a query with
a view
 The query is the one being asked
 The views are the MSL templates for the wrappers
 Some of the views may actually require parameters, e.g.,
an author name, before they’ll return answers
 Common for web forms (see Amazon, Google, …)
 XQuery functions (XQuery’s version of views) support
parameters as well, so we’ll see these in action
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A Wrapper Definition in MSL
Wrappers have templates and binding patterns ($X) in
MSL:
B :- B: <book {<author $X>}>
// $$ = “select * from book where author=“ $X //
 This reformats a SQL query over Book(author, year, title)
In XQuery, this might look like:
declare function GetBook($x AS xsd:string) as book {
for $b in
title
sql(“Amazon.DB”,
“select * from book where author=‘” + $x +”’”)
return <book>{$b/title}<author>$x</author></book>
}
…
…
book
author
…
The union of GetBook’s results is unioned with others to form the view Mediator()
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How to Answer the Query
Given our query:
for $b in Mediator()/book
where $b/title/text() = “DB2 UDB” and
$b/author/text() = “Chamberlin”
return $b
Find all wrapper definitions that:
 Contain output enough “structure” to match the
conditions of the query
 Or have already tested the conditions for us!
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Query Composition with Views
We find all views that define book with author and
title, and we compose the query with each:
book
declare function GetBook($x AS xsd:string) as book {
for $b in
sql(“Amazon.DB”,
author
title
“select * from book where author=‘” + $x + “’”)
return <book> {$b/title} <author>{$x}</author></book>
}
…
…
for $b in Mediator()/book
where $b/title/text() = “DB2 UDB” and
$b/author/text() = “Chamberlin”
return $b
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Matching View Output to
Our Query’s Conditions
 Determine that $b/book/author/text()  $x by matching the
pattern on the function’s output:
declare function GetBook($x AS xsd:string) as book {
for $b in
sql(“Amazon.DB”,
“select * from book where author=‘” + $x + “’”)
return <book>{ $b/title }
<author>{$x}</author></book>
}
book
title
let $x := “Chamberlin”
for $b in GetBook($x)/book
where $b/title/text() = “DB2 UDB”
return $b
…
author
…
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The Final Step: Unfolding
let $x := “Chamberlin”
for $b in (
for $b’ in
sql(“Amazon.com”,
“select * from book where author=‘” + $x + “’”)
return <book>{ $b/title }<author>{$x}</author></book>
)/book
where $b/title/text() = “DB2 UDB”
return $b
 How do we simplify further to get to here?
for $b in sql(“Amazon.com”,
“select * from book where author=‘Chamberlin’”)
where $b/title/text() = “DB2 UDB”
return $b
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Virtues of TSIMMIS
 Early adopter of semistructured data, greatly
predating XML
 Can support data from many different kinds of sources
 Obviously, doesn’t fully solve heterogeneity problem
 Presents a mediated schema that is the union of
multiple views
 Query answering based on view unfolding
 Easily composed in a hierarchy of mediators
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Limitations of TSIMMIS’ Approach
Some data sources may contain data with certain
ranges or properties
 “Books by Aho”, “Students at UPenn”, …
 If we ask a query for students at Columbia, don’t want to
bother querying students at Penn…
 How do we express these?
Mediated schema is basically the union of the various
MSL templates – as they change, so may the mediated
schema
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An Alternate Approach:
The Information Manifold (Levy et al.)
When you integrate something, you have some
conceptual model of the integrated domain
 Define that as a basic frame of reference, everything else
as a view over it
 “Local as View”
May have overlapping/incomplete sources
 Define each source as the subset of a query over the
mediated schema
 We can use selection or join predicates to specify that a
source contains a range of values:
ComputerBooks(…)  Books(Title, …, Subj), Subj = “Computers”
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The Local-as-View Model
The basic model is the following:
 “Local” sources are views over the mediated schema
 Sources have the data – mediated schema is virtual
 Sources may not have all the data from the domain –
“open-world assumption”
The system must use the sources (views) to answer
queries over the mediated schema
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Query Answering
Assumption: conjunctive queries, set semantics
Suppose we have a mediated schema:
author(aID, isbn, year), book(isbn, title, publisher)
Suppose we have the query:
q(a, t) :- author(a, i, _), book(i, t, p), t = “DB2 UDB”
and sources:
s1(a,t)  author(a, i, _), book(i, t, p), t = “123”
…
s5(a, t, p)  author(a, i, _), book(i,t), p = “SAMS”
We want to compose the query with the source mappings – but
they’re in the wrong direction!
 Yet: everything in s1, s5 is an answer to the query!
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Answering Queries Using Views
Numerous recently-developed algorithms for these
 Inverse rules [Duschka et al.]
 Bucket algorithm [Levy et al.]
 MiniCon [Pottinger & Halevy]
 Also related: “chase and backchase” [Popa, Tannen, Deutsch]
Requires conjunctive queries
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Summary of Data Integration
Local-as-view integration has replaced global-as-view as the standard
 More robust way of defining mediated schemas and sources
 Mediated schema is clearly defined, less likely to change
 Sources can be more accurately described
Methods exist for query reformulation, including inverse rules
Integration requires standardization on a single schema
 Can be hard to get consensus
 Today we have peer-to-peer data integration, e.g., Piazza [Halevy et al.], Orchestra [Ives
et al.], Hyperion [Miller et al.]
Some other aspects of integration were addressed in related papers
 Overlap between sources; coverage of data at sources
 Semi-automated creation of mappings and wrappers
Data integration capabilities in commercial products: BEA’s Liquid Data, IBM’s DB2
Information Integrator, numerous packages from middleware companies
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Initial Lecture