Use zero-length characters to indicate that there is no data
To indicate that data does not exist in the Microsoft Access database, you can enter a zero-length string in the Text or Notes field (zero-length string: a string without characters. You can use a zero-length string to indicate that you know that the field has no value. The way to enter a zero-length string is to type two double quotes ("") that have no spaces between each other.). In a Microsoft Access project connected to the Microsoft SQLServer database (Microsoft Access project: Access file that is connected to the Microsoft SQLServer database and used to create client/server applications. The project file does not contain any data or data-defined objects (such as tables or views).) You can enter a zero-length string in its data type (Data type: Field characteristics that determine the type of data that a field can have. Data types include Boolean, Integer, Long, Currency, Single, Double, Date, String, and Variant (default).) Enter a zero-length string in a field that is varchar or nvarchar.
Open a data table (DataTable view: A window that displays a table, form, query, view, or stored procedure in row-column format. In the DataTable view, you can edit fields, add and delete data, and search for data.), or open a form in the Form view (Form view: a window that displays a form to display or accept data. The Form view is the primary way to add and modify data in a table. In this view, you can also change the design of the form.).
To enter a zero-length string in the field, type double quotes ("") without spaces in the middle.
Even though the double quotes disappear when moved to another field or object, the field is still a zero-length string.
Difference between Null value and zero-length string (MDB)
/*Note The information in this topic applies only to Microsoft Access databases (.mdb). */
When viewing data containing Null (Null: can be entered in a field or used for expressions and queries to indicate missing or unknown data. In Visual Basic, the Null keyword represents a Null value. Some fields (such as primary key fields) cannot contain Null values.) Value and a zero-length string (zero-length string: a string without characters. You can use a zero-length string to indicate that you know that the field has no value. The way to enter a zero-length string is to type two double quotes ("") with no spaces between each other.), the fields look the same, neither with any value. However, expressions (expression: any combination of arithmetic or logical operators, constants, functions and field names, controls, and properties, and the result is a single value. The expression can perform calculations, manipulate characters, or test data.) to distinguish between Null values and zero-length strings.
In Design view (Design view: A window that displays the design of a database object (including: tables, queries, forms, macros, and data access pages). In Design view, you can create new database objects and modify the design of an existing database object.).