According to the present invention there is provided a user interface for mapping data sets to a target data structure. Each data set comprises data segments separated by a separator. The target data structure comprises target data fields. The user interface comprises: 1) a segment display section in which data segments of one data set are displayed; 2) a target display section in which possible target fields to which data segments can be associated/mapped are displayed; 3) a selection device for associating a predetermined data segment with a target field; and 4) a contents preview display section in which the different values contained in one selected data segment of all data sets is displayed. Furthermore, there is provided a method and computer system.