greatest returns the greatest of the list of expr's.
All expr's after the first are implicitly converted to the
datatype of the first expr before the comparison. Oracle compares
the expr's using nonpadded comparison semantics. Character
comparison is based on the value of the character in the database
character set. One character is greater than another if it has a higher
character set value. If the value returned by this function is character
data, then its datatype is always VARCHAR2.