NVL replaces a null with a string.
NVL returns the replacement string when the base expression is null, and the value of the base expression when it is not null.
To replace an expression with one value if it is null and a different value if it is not, use
Data type of the first argument
expr1 is the base expression that is evaluated.
expr2 is the replacement string that is returned when
expr1 is null.
NVL('First String', 'Second String') returns the value
NVL(null, 'Second String') returns the value