Constraint Null Value

I am is Alex, and I m developing a product with PostgreSQL 8.1. I have a
problem. This problem is that i have some unique constraints with
multi-columns. The problem is that there is some that of the check
constraint condition that can be null. As it is described in section
5.3.3, null of one row is different from the null of other row, so we
can have two entries with same values inserted because of null value is
not a considered equal in this comparison.

Is there any administration syntax to change this behavior so null
values are treated as same value?

Thank you very much.

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