what is the function of nvl?
The NVL function. You can use the NVL function to convert an expression that evaluates to NULL to a value that you specify. The NVL function accepts two arguments: the first argument takes the name of the expression to be evaluated; the second argument specifies the value that the function returns when the first argument evaluates to . If the first argument does not evaluate to NULL, the function returns the value of the first argument.