Why does = NULL not work in Oracle?
Take a look at PSOUG's notes on NULL.  As Fabricio Araujo hinted, NULL is not really a value like the number 4 or string 'bacon strips'.  In fact, NULL is untyped in the SQL language, which is why you cannot validly use it in an equality comparison.  You need the special IS [NOT] NULL syntax to check if a value is NULL or not.