Goal of Data Integration - provide uniform access to heterogeneous data sources in some domain.
- data from all data sources are federated into one main warehouse (using ETLs)
- the queries are issued to this federated storage
- data remains in the data sources
- sometimes called "virtual data integration"
- better for the Web - there are many DBs, and we would like to find something, no matter what DB provides it
- also better if you want to access "fresh" data
- but way harder to implement - need to transform data during the query time