Oracle 18c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE SalesOrder (SalesOrderID, Customer, SoldDate, ShipDate) AS
SELECT 1,	'Customer1', DATE '2022-07-12', DATE '2022-07-12' FROM DUAL UNION ALL
SELECT 2,	'Customer1', DATE '2022-07-12', DATE '2022-07-12' FROM DUAL UNION ALL
SELECT 3,	'Customer2', DATE '2022-07-12', DATE '2022-07-12' FROM DUAL;
3 rows affected

CREATE TABLE Lines (LineItemID, LineNumber, SalesOrderID, Product, Price) AS
SELECT 1,	1,	1,	'Eggs',	 1.00 FROM DUAL UNION ALL
SELECT 2,	2,	1,	'Milk',	 1.00 FROM DUAL UNION ALL
SELECT 3,	1,	2,	'Bread', 1.00 FROM DUAL UNION ALL
SELECT 4,	2,	2,	'Eggs',	 1.00 FROM DUAL UNION ALL
SELECT 5,	3,	2,	'Milk',	 1.00 FROM DUAL UNION ALL
SELECT 6,	1,	3,	'Eggs',	 1.00 FROM DUAL;
6 rows affected

SELECT JSON_OBJECT(
         KEY 'salesOrderId' VALUE s.salesOrderId,
         KEY 'customer'     VALUE s.customer,
         KEY 'soldDate'     VALUE s.soldDate,
         KEY 'shipDate'     VALUE s.shipDate,
         KEY 'lineItems'    VALUE l.lineItems FORMAT JSON
         RETURNING CLOB
       ) AS salesorder
FROM   salesorder s
       CROSS JOIN LATERAL (
         SELECT JSON_ARRAYAGG(
                  JSON_OBJECT(
                    KEY 'lineItemId' VALUE l.lineItemid,
                    KEY 'lineNumber' VALUE l.lineNumber,
                    KEY 'product'    VALUE l.product,
                    KEY 'price'      VALUE l.price
                  )
                  ORDER BY l.lineNumber
                  RETURNING CLOB
                ) AS lineItems
         FROM   lines l
         WHERE  l.salesorderid = s.salesorderid
       ) l


SALESORDER


{"salesOrderId":1,"customer":"Customer1","soldDate":"2022-07-12T00:00:00","shipDate":"2022-07-12T00:00:00","lineItems":[{"lineItemId":1,"lineNumber":1,"product":"Eggs","price":1},{"lineItemId":2,"lineNumber":2,"product":"Milk","price":1}]}

{"salesOrderId":2,"customer":"Customer1","soldDate":"2022-07-12T00:00:00","shipDate":"2022-07-12T00:00:00","lineItems":[{"lineItemId":3,"lineNumber":1,"product":"Bread","price":1},{"lineItemId":4,"lineNumber":2,"product":"Eggs","price":1},{"lineItemId":5,"lineNumber":3,"product":"Milk","price":1}]}

{"salesOrderId":3,"customer":"Customer2","soldDate":"2022-07-12T00:00:00","shipDate":"2022-07-12T00:00:00","lineItems":[{"lineItemId":6,"lineNumber":1,"product":"Eggs","price":1}]}