The proper way to implement unique constraint that allows multiple NULL values in SQL Server

Question

I need 1 column in the table to hold unique non-null values or NULL. TSQL UNIQUE constraint treats 2 NULLs as equal, so I cannot make column unique.
What is the right way to handle this problem?
After doing some research, I found 2 methods which seem correct to me, but I cannot determine which one is better.
The first which is not applied to all cases:

CREATE TABLE test (id INT NOT NULL IDENTITY(1,1) PRIMARY KEY, 
   null_or_unique_id INT, unique_key AS  
(CASE WHEN [null_or_unique_id] IS NULL THEN -(1)*[id] 
 ELSE [null_or_unique_id] END), UNIQUE(unique_key ));

It works but requires all allowed values of null_or_unique_id to be non-negative (that's ok in my case)
The second one :

 CREATE VIEW test_view WITH SCHEMABINDING AS
 SELECT [null_or_unique_id] FROM dbo.test WHERE [null_or_unique_id] IS NOT NULL;
 GO
 CREATE UNIQUE CLUSTERED INDEX byNullOrUniqueId 
 ON dbo.test_view([null_or_unique_id]);

Surely, it's also possible to implement the desired functionality with triggers, but I think trigger solution will create more overhead then any of mentioned above.

What is the best practice for such a case?
Thanks for your answers.

Answer 1

4 ways:

• Filtered index (SQL Server 2008) <- recommended based on your tags
• Trigger (mentioned)
• Indexed view (in your question)
• Unique constraint/index with computed column (in your question)

Answer 2

SQL 2008 allows you to define a filtered index - essentially an index with a WHERE clause - see Phil Haselden's asnwer to this question for MSDN links and an example.