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Set properties that control blank values (MDB)

Note  The information in this topic applies only to a Microsoft Access database (.mdb).

You can specify the types of blank values that are allowed in a field by setting different combinations of the field's Required and AllowZeroLength properties. Microsoft Access allows you to distinguish between two kinds of blank values: Null values and zero-length strings. The Required property determines whether users can leave a field blank, resulting in a Null value. The AllowZeroLength property determines whether Text, Memo, or Hyperlink fields can contain a zero-length string.

1. In table Design view, in the upper portion of the window, click the field for which you want to allow or prevent blank values.

2. In the lower portion of the window, do one of the following:

   To allow blank values when you don't need to distinguish blank values that indicate unknown data from blank values that indicate you know there's no value, set both the Required and AllowZeroLength properties to No.

   To prevent users from leaving a field blank, set the Required property to Yes and the AllowZeroLength property to No.

   To allow both types of blank values, so that you can distinguish blank values that indicate unknown data from blank values that indicate you know there's no data, set the Required property to No and the AllowZeroLength property to Yes.

   To allow blank values in a field only when you know that there is no data, set both the Required property and the AllowZeroLength property to Yes. In this case, the only way to leave a field blank is to type double quotation marks with no space between them, or press the SPACEBAR to enter a zero-length string.