The NULL value
•NULL marks an attribute undefined
•NULL is a valid value of ANY data type
•Unspecified attributes get the NULL value
•NULL is not equal to any value
but
NULL is equivalent to NULL
•The function NVL(x,y) returns y if x equivalent NULL, else it returns x
•
Arithmetical expressions with NULL values return NULL.
The NVL function NVL( x, 0 ) returns 0 is x is NULL, else it returns x.