In SQL, NULL represents a missing/unknown value. The basic comparison
operators like =, <>, <, >, IN, etc return an UNKNOWN result when NULLs are
compared. IS NULL / IS NOT NULL are special types of comparison for
verifying the presence or absence of NULLs.

Read about NULLs and three-value logic in Books Online:
http://msdn.microsoft.com/libr*ary/e...d_02_8p*wy.asp

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David Portas
SQL Server MVP
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