Historical projection is an analogue of conventional projection, where component(s) of tuples are retained, in this case, components of the time(s) the data elements were valid. One fundamental question is whether the derived intervals must be subsets of the underlying intervals. A second design issue is whether the target list in SQL should be extended, or whether a new clause, such as the valid clause in TQuel, is preferred. The subject of temporal joins, such as time intersection, time union, and temporal outer joins, also needs to be addressed here.