Stewart "Arturius" Campbell (12/20/2012)

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IMHO, I think the correct answer here will be "It depends".
It depends on whether filtered indexes are utilised or not, etc.

However, it could be that the PO intended that the use of filtered index(es) is implied (or should be assumed)...

SQLRNNR (12/20/2012)

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I disagree that "It depends" with this question. It was asked if it was simply possible. No need to consider filtered indexes, you just need to know if there is any possible way to insert multiple nulls into a unique constraint - imho.

SQLRNNR (12/20/2012)

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I disagree that "It depends" with this question. It was asked if it was simply possible. No need to consider filtered indexes, you just need to know if there is any possible way to insert multiple nulls into a unique constraint - imho.

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);

dawryn (12/20/2012)

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...
Besides, only one occurrence of Nulls in all fields is allowed.

Gopi S (12/20/2012)

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Technically there is no difference between Unique Index and Unique Constraint. Even though syntax are different the effect is the same.
Unique Constraint creates Unique Index to maintain the constraint to prevent duplicate keys. Unique Index also creates index that are physical structure that maintain uniqueness. It is a convenient way to enforce a Unique Constraint for SQL Server.
In above case "Unique Constraint" allows one "NULL".

Gopi S (12/20/2012)

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So far Jason is following me