Call this member function to set the field to a Null value.
void SetFieldValueNull(
   int nIndex 
);
void SetFieldValueNull(
   LPCTSTR lpszName 
);
Parameters
- nIndex 
 The index of the field in the recordset, for lookup by zero-based index.
- lpszName 
 The name of the field in the recordset, for lookup by name.
Remarks
C++ NULL is not the same as Null, which, in database terminology, means "having no value."
For related information, see the topics "Field Object" and "Value Property" in DAO Help.
Requirements
Header: afxdao.h