What is the difference between the function nvl() and the expression  $(if Field=null then "" else Field)

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    Customer needs to retrieve a specifically defined value in the event that a field has no value\entry.
    When trying to use the function nvl ([value to be null], [value to return]), but the field is not being populated.
    They would like to know why this is not populating the field  while the expression $(if [Field]=null then "" else [Field])


    The function nvl () first determines if the  value of the first parameter is null. If it is, it returns the value of the second parameter.
    This value however is only being returned, it is not a value that is saved in the field.

    When using the expression $(if Field=null then "" else Field) the expression first evaluates the field value, and -if empty- the alternative value is being populated and saved in the field.