Data integration systems harmonize data from multiple sources into a single coherent representation. The goal is to provide an integrated view over all the data sources of interest and to provide a uniform interface to access all of these data. The access to the integrated data is usually in the form of querying rather than updating the data.
The data sources to be integrated may belong to the same enterprise or may be arbitrary sources on the web. Most of the time, each of the sources is independently designed for autonomous operation. Also, the sources are not necessarily databases; they may be legacy systems (old and obsolescent systems that are difficult to migrate to a modern technology) or structured/unstructured files with different interfaces. Data integration requires that the differences in modeling, semantics and capabilities of the sources together with the possible inconsistencies be resolved.