Oracle XML ORA-00978: nested group function without GROUP BY - oracle

Question

I am trying to generate XML using Oracle's XML functions but I keep getting the ORA-00978: nested group function without GROUP BY error message.
I am new to XML and also Oracle's XML functions so it seems I am missing something major however when looking at examples online I can't understand what I am doing wrong.
SQL:
select
xmlelement("apiRequest",
xmlelement("orders",
xmlagg(
xmlelement("order",
xmlelement("no", orders.order_no),
xmlelement("date", orders.date),
xmlelement("orderItems",
xmlagg(
xmlelement("orderItem",
xmlelement("position", order_items.item_position)
)
)
)
)
)
)
)
as xml
from
...
Desired output:
<apiRequest>
<orders>
<order>
<no>1</no>
<date>04/03/2010</date>
<orderItems>
<orderItem>
<position>1</position>
</orderItem>
<orderItem>
<position>2</position>
</orderItem>
<orderItem>
<position>3</position>
</orderItem>
<orderItem>
<position>4</position>
</orderItem>
</orderItems>
</order>
<order>
<no>2</no>
<date>04/03/2010</date>
<orderItems>
<orderItem>
<position>1</position>
</orderItem>
<orderItem>
<position>2</position>
</orderItem>
<orderItem>
<position>3</position>
</orderItem>
<orderItem>
<position>4</position>
</orderItem>
</orderItems>
</order>
</orders>
</apiRequest>

Answer 1

When you do a nested aggregation, you need a GROUP BY clause. The first level of aggregation is at the GROUP BY level, then the result of that is aggregated again.
To illustrate, I used the ORDERS and ORDER_ITEMS tables from the OE schema that Oracle provides.
select
xmlelement("apiRequest",
xmlelement("orders",
xmlagg(
xmlelement("order",
xmlelement("no", order_id),
xmlelement("date", oe.orders.order_date),
xmlelement("orderItems",
xmlagg(
xmlelement("orderItem",
xmlelement("position", oe.order_items.line_item_id)
)
)
)
)
)
)
)
as xml
from oe.orders join oe.order_items using(order_id)
group by order_id, order_date;
The other answer uses a scalar subquery, which I would avoid in this case because it executes one recursive SQL for each order instead of simply joining.

Answer 2

You could put second xmlagg in a subquery, it gives desired result:
select
xmlelement("apiRequest",
xmlelement("orders",
xmlagg(
xmlelement("order",
xmlelement("no", orders.order_no),
xmlelement("date", orders.order_date),
xmlelement("orderItems",
(select xmlagg(
xmlelement("orderItem",
xmlelement("position", order_items.item_position)
)
)
from order_items where order_no = orders.order_no)
))))) as xml
from orders
dbfiddle