SQLRNNR (12/20/2012)

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I don't disagree with the correct answer. I disagree with the explanation. I also do disagree with many of the complaints thus far about the question. You can most certainly have multiple null values in a unique constraint. Don't constrain yourselves to a narrow scope of a constraint on a single column - where you can have only one null value.

ALTER TABLE sometest

ADD CONSTRAINT someconstraint UNIQUE (testid,col1,col2);