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Determines if a specified field or expression is blank.
An expression of any data type.
ISBLANK( ) returns true if a specified expression is blank or null; false if it contains data. A field is blank if it has never contained a value or if you used the BLANK command on it. ISBLANK( ) returns a different result from EMPTY( ) when used on numeric fields; ISBLANK( ) differentiates between zero and blank values, while EMPTY( ) does not.
ISBLANK( ) is especially useful when performing functions such as averaging, since it ensures that blank values are not included in the calculation. If you don't need to differentiate between 0 or blank values in numeric fields, you can use either ISBLANK( ) or EMPTY( ).