ISNULL( )
Tests if a value in a specified field is a null value. A null value means that no value exists.
Syntax
ISNULL(source)
Argument
source
The field in which to check for null values.
Returns
True if a value in the specified field is a null value; returns false otherwise.
Example
The following example uses ISNULL( ) in conjunction with the IF( ) function to test for null values in the BirthDate field. If there is a null value, display No date specified; otherwise display the BirthDate value.
IF(ISNULL([BirthDate]), "No date specified", [BirthDate])

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