Unique record to display, with ORDER BY

Question

Table order
Order_Number, Cust_Name
222, XYZ
223, XYZ
224, ABC
Table Product
Order_Number, p_name
222, p1
222, p2
222, p3
223, p1
223, p4
here i need to display only 2 records (1 record for one order number) but with ORDER BY as below
SELECT Order_Number, Cust_Name
FROM order o
INNER JOIN Product p ON (o.Order_Number = p.Order_Number)
WHERE o.Cust_Name = 'XYZ'
ORDER BY p_name;
1) If i put DISTINCT, below error
SQL Error: ORA-01791: not a SELECTed expression
01791. 00000 - "not a SELECTed expression"
2) If i put GROUP BY, below error
SQL Error: ORA-00979: not a GROUP BY expression
00979. 00000 - "not a GROUP BY expression"
please help me to form the query

Answer 1

Try this
select o.order_number, o.cust_name
  from (
select o.*,p.*, row_number() over(partition by o.order_number order by p.p_name)
  from order o
  join product p
    on o.order_number = p.order_number
where o.cust_name = 'XYZ'
where rno = 1