Hugo Kornelis (10/28/2010)

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Good question, but a small (yet significan) mistake in the explanation.
NULL does not represent the value 'UNKNOWN'.

SanDroid (10/28/2010)

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Hugo, perhaps you could ask M$ to update the books online to match your opinion?

<Quote from BOL>
When SET ANSI_NULLS is ON, a comparison in which one or more of the expressions is NULL does not yield either TRUE or FALSE; it yields UNKNOWN. This is because a value that is unknown cannot be compared logically against any other value. This occurs if either an expression is compared to the literal NULL, or if two expressions are compared and one of them evaluates to NULL. For example, the following comparison always yields UNKNOWN when ANSI_NULLS is ON:

Copyytd_sales > NULL

The following comparison also yields UNKNOWN any time the variable contains the value NULL:

ytd_sales > @MyVariable

<Quote from BOL>

Youre comment is correct with one small but significant correction.
When ANSI_NULLS is OFF NULL is not value UNKNOWN.

Hugo Kornelis (10/28/2010)

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[quote]I wrote that NULL does not represent unknown.

SanDroid (10/28/2010)

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I understand that is what you wrote.
I thought maybe you would look at the referenced article before responding.

Let me grab another cut and paste of the BOL that was referenced for the question.
"The value NULL means the data value for the column is unknown or not available."

That is the very first sentance from the Books online:
http://msdn.microsoft.com/en-us/library/ms191270%28v=SQL.100%29.aspx

Now I understand that the Books Online have often been wrong.
Do you believe they are in this instance?