Let's confirm that you have a table in the Access database (not attached
from a different program), and in table design you have set the Required
property of the field to Yes.
If you do that, Access will not allow you to add any more records where the
field contains null.
Notes:
a) There may be some existing records where the field is null, if they were
created before you set the field's Required property.
b) If the field is of type Text, Memo, or Hyperlink, you also need to set
the Allow Zero Length property to No so it cannot contain a zero-length
string.
c) If you put a close button on your form, and use the Close action (in a
macro) or method (in code), you don't get any warning when you close the
form if the required field is Null. However, you don't get the record saved
either: Access just silently loses the entry. Details:
http://allenbrowne.com/bug-01.html
d) If you do want to check a field for null in code, use the BeforeUpate
event of the *form* (not controls.) This allows you to test and give a
custom message, or a warning you can allow the user to override. But it is
not necessary if you have set the Required property of the field to Yes in
