A computer program product is provided that uses data examples as a basis for understanding and refining declarative schema mappings. The system of the present invention identifies a set of intuitive operators for manipulating examples including establishing value correspondences, data linking, data trimming, data walking, and data chasing. These operators allow a user to follow and refine an example by walking through a data source. In addition, these operators can identify a large class of schema mappings and distinguish effectively between alternative schema mappings. With these operators, a user is able to quickly and intuitively build and refine complex data transformation queries that map one data source into another while continuously verifying that the mapping is accurate and appropriate.