Data items cannot be combined indiscriminately. The absence or presence of a key data item in a view determines which other data items are allowed in the view.
If a view contains a key data item, all result data items in the view must be valid for the key data item. Also, for each result data item in a view, all required keys for that result data item must be in the view.
If a view does not contain a key data item, it can include any data item that does not require a key.
