Database Reference
In-Depth Information
1
Data exchange by example
Data exchange is the problem of finding an instance of a target schema, given an instance
of a source schema and a specification of the relationship between the source and the target.
Such a target instance should correctly represent information from the source instance un-
der the constraints imposed by the target schema, and should allow one to evaluate queries
on the target instance in a way that is semantically consistent with the source data.
Data exchange is an old problem that re-emerged as an active research topic recently
due to the increased need for exchange of data in various formats, often in e-business
applications.
The general setting of data exchange is this:
query
Q
mapping
M
source
S
target
T
We have fixed source and target schemas, an instance
S
of the source schema, and a map-
ping
that specifies the relationship between the source and the target schemas. The goal
is to construct an instance
T
of the target schema, based on the source and the mapping,
and answer queries against the target data in a way consistent with the source data.
The goal of this introductory chapter is to make precise some of the key notions of
data exchange: schema mappings, solutions, source-to-target dependencies, and certain
answers. We do it by means of an example we present in the next section.
M
1.1 A data exchange example
Suppose we want to create a database containing three relations:
•
ROUTES(flight#,source,destination)
This relation has information about routes served by several airlines: it has a
flight#
attribute (e.g., AF406 or KLM1276), as well as
source
and
destination
attributes
(e.g., Paris and Santiago for AF406).
