In data integration systems there is a problem of answering queries through a target schema, given a set of mappings between source schemas and the target schema, and given that the data is at the sources. This is of special importance when integrated sources, e.g. from Web data repositories, have overlapping data and its merging is necessary. We propose a language for specifying a class of mappings between source and target schemas, and design rewriting rules which reformulate a target query to a query over data sources based on the mappings.