Hugo Kornelis (8/22/2013)

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palotaiarpad (8/22/2013)

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BOL: However, you can change this behavior by changing the setting of CONCAT_NULL_YIELDS_NULL for the current session.

The right answer is: it depends. 😀

Maybe you should specify WHERE in Books Online you found that quote. I am pretty sure that it's on a page that does not describe the CONCAT() function, but other methods of string concatenation. CONCAT() is not affected by this setting.