Data exchange, as the name suggests, is the problem of exchanging data between different databases that have different schemas. One often needs to exchange data between existing legacy databases, whose schemas cannot be easily modified, and thus one needs to specify rules for translating data from one database to the other. These rules are known as schema mappings. Once a source database and a schema mapping are given, one needs to transfer data to the target, i.e., construct a target database. And once the target database is constructed, one needs to answer queries against it.

This problem is quite old; it has been studied, and systems have been built, but it was done in a rather ad hoc way. A systematic study of the problem of data exchange commenced with the 2003 paper “Data exchange: semantics and query answering” by Fagin, Kolaitis, Miller, and Popa, published in the proceedings of the International Conference on Database Theory. A large number of followup papers appeared, and for a while data exchange was one of the most active research topics in databases. Foundational questions related to data exchange largely revolved around three key problems:

1. how to build a target solution;

2. how to answer queries over target solutions; and

3. how to manipulate schema mappings themselves.

The last question is also known under the name of metadata management, since mappings represent metadata, rather than data in the database.