Unique constraint and NULL values

I want to enforce such a constraint on a column that would ensure that the values be all unique, but this wouldn't apply to NULL values. (I.e. there may be more than one NULL value in the column.) How can I achieve this?
I suppose I would get the most-hated "table/view is changing, trigger/function may not see it" error if I tried to write a trigger that checks the uniqueness of non-null values upon insert/update. Received on Thu Oct 21 2004 - 07:45:56 CDT