Here is some further info on Access Nulls and Blanks...
About blank values in fields
Microsoft Access allows you to distinguish between two kinds of blank values: Null values and zero-length strings. In some situations, a blank value indicates that the information might exist but is not currently known. In other situations, a blank value indicates that the field does not apply to a particular record.
For example, if a Customers table contains a Fax Number field, you can leave the field blank if you're unsure of the customer's fax number, or whether the customer even has a fax. In this case, leaving the field blank enters a Null value, which means you don't know what the value is. If you later determine that the customer doesn't have a fax, you can enter a zero-length string in the field to indicate that you know there is no value.
Indicate nonexistent data with a zero-length string
To indicate nonexistent data in a Microsoft Access database, you can enter a zero-length string into a Text or Memo field. In a Microsoft Access project that is connected to a Microsoft SQL Server database, you can enter a zero-length string in a field whose data type is varchar or nvarchar.
Open a datasheet, or open a form in Form view.
To enter a zero-length string in the field, type two double quotation marks without a space in between (" ").
The field remains a zero-length string, even though the quotation marks disappear when you move to another field or object.
