Kod:
Private Sub ProductName_BeforeUpdate(Cancel As Integer)
If(Not IsNull(DLookup("[ProductName]", _
"Products", "[ProductName] ='" _
&; Me!ProductName &; "'"))) Then
MsgBox "Product has already been entered in the database."
Cancel = True
Me!ProductName.Undo
End If
End Sub
konusunda bahsettiğim arşivden buldum. Bur