I have a business rule which gives me rahter big challenges to "melt in SQL": At one "point in time" only one version may be active. Simplified: CREATE TABLE sample ( id_field int4, value text, validfrom timestamp, validto timestamp ) within one timespan, every id_field has to be unique. so, this is a set of legal data: 1, 'fire', -infinity , 2005-09-01 21:02:15.078 1, 'water', 2005-09-01 21:02:15.078, infinity this is an illegal set of data: 1, 'fire', -infinity , 2005-09-01 21:02:15.078 1, 'water', 2005-05-01 12:15:15.078, infinity because between 2005-05-01 12:15:15.078 and 2005-09-01 21:02:15.078 there are 2 values for id_field 1 As a first measure I have made a unique key on (id_field , validfrom), because the "timespans" are created by update rules of a view, so that I am quite sure that there are no overlapping ... as long as nobody touches the table by himself. But it is not really fitting; and manual editing of the table can disturb it. So, ist there some constraint creatable to make sure that "in each point of time" each id only exists once? Harald -- GHUM Harald Massa persuadere et programmare Harald Armin Massa Reinsburgstraße 202b 70197 Stuttgart 0173/9409607 - PostgreSQL - supported by a community that does not put you on hold
