General Functions

The objective of the general NULL handling functions is to replace the NULL values with an alternate value.

NVL - The NVL function substitutes an alternate value for a NULL value.
NVL2 - As an enhancement over NVL, Oracle introduced a function to substitute value not only for NULL columns values but also for NOT NULL columns.
NULLIF - The NULLIF function compares two arguments expr1 and expr2. If expr1 and expr2 are equal, it returns NULL; else, it returns expr1. First argument can't be NULL.
COALESCE - COALESCE function, a more generic form of NVL, returns the first non-null expression in the argument list.