UNIQUE OR NULL

Hy Guys, I have a problem that seems to be very simple but I think it is not. I have a column that may contain NULL values but if there is a value different from NULL this has to be unique. I.e. in other words no value except of NULL may occur more than once. Question: How do I explain this in INFORMIX by a constraint or by other means? I would NOT prefer a trigger solution but declarative means, of course. In practice, I think this is a very important issue. Very often you have columns that have to be unique if the value is known but on the other hand it is possible that the value is not known when the row is inserted. Any ideas? Achim -- Achim Reiners Schleupen Computersysteme GmbH & Co EDV fuer das Gesundheitswesen KG Richard-Byrd-Str. 35 ** Tel.: (49) 221 - 956436-32 D-50829 Koeln ** EMail: ar@schleupen-gw-koeln.de