It takes two parametes like,
NULLIF (param1,param2)
and returns NULL if both param1 and param2 are equal.
NULLIF returns the first parameter if the two parameters are not equivalent
NOTE:
param1 and param2 can be a constant, column name, function, subquery, or any combination of arithmetic, bitwise, and string operators.
NULLIF is somehow similar to CASE function.
http://www.bloglines.com/blog/sabah-irfan?id=41
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