When you want to define different blocks of data, some hierarchical, some not, you can do that by using the cursor expressions in SQL. An example of a query looks like this:
cursor(select o.order_total, order_name,
cursor(select p.product_name, i.quantity, i.unit_price
from demo_order_items i, demo_product_info p
where o.order_id = i.order_id
and i.product_id = p.product_id
from demo_orders o
where c.customer_id = o.customer_id
from demo_customers c
In the above query you see you can nest the cursor expressions. But you can also define the cursors next to each other. We use this technique a lot when defining where the data comes from in the APEX Office Print (AOP) APEX plugin:
In Oracle Application Express 18.1 there's a small bug (Bug 28298260 - REGRESSION: SQL QUERY CONTAINING CURSOR EXPRESSION CAN'T BE PARSED) that when you validate the query you get "ORA-22902: CURSOR expression not allowed".
The APEX Dev team already fixed it - you can download from Oracle Support the bundle PSE patch #28128115. Once applied everything is validating correctly again.
AOP also supports REST web services, and some people define those in ORDS (Oracle REST Data Services). Depending the version of ORDS you might get the same error: "Error during evaluation of resource template: GET test/cursor/, SQL Error Code: 22,902, SQL Error Message: ORA-22902: CURSOR expression not allowed"
This doesn't mean your environment can not use the cursor syntax, you just have to set the pagination size to 0 and it's fixed.
In the latest version of ORDS (18.2) you get by default the 500 error without the error number:
Fix is the same, set pagination to 0 and you are good to go.