Thank you Maciej Los,
I improved your answer to this:
when choose ItemId combobox, NCode combobox will choose the correct item according to ItemId value, and vice versa.
Private Sub ItemId_Change()
Dim sSQL As String
sSQL = "SELECT Items.NationalCode, Items.ItemId " & "FROM Items" & " WHERE Items.ItemId = " & Me.ItemId.Value
Me.NCode.RowSource = sSQL
Me.NCode = Me.NCode.ItemData(0)
End Sub
Private Sub ItemId_GotFocus()
Dim sSQL As String
sSQL = "SELECT Items.GenericName, Items.ItemId" & " FROM Items"
Me.ItemId.RowSource = sSQL
End Sub
Private Sub NCode_Change()
Dim sSQL As String
sSQL = "SELECT Items.GenericName, Items.ItemId" & " FROM Items" & " WHERE Items.ItemId = " & Me.NCode.Value
Me.ItemId.RowSource = sSQL
Me.ItemId = Me.ItemId.ItemData(0)
End Sub
Private Sub NCode_GotFocus()
Dim sSQL As String
sSQL = "SELECT Items.NationalCode, Items.ItemId" & " FROM Items"
Me.NCode.RowSource = sSQL
End Sub