在while循环中而不是 null 应该有一个属于此订单的订单行列表。
查询：

SELECT orders.order_id, orders.orderNumber, orderRow.itemName, orderRow.quantity, orderRow.price
FROM orders
LEFT JOIN orderRow
ON orders.order_id=orderRow.order_id

方法：

public List<Order> findAllOrders() {

        String sql = "SELECT orders.order_id, orders.orderNumber, orderRow.itemName, orderRow.quantity, orderRow.price\n" +
                "FROM orders\n" +
                "LEFT JOIN orderRow\n" +
                "ON orders.order_id=orderRow.order_id";

        try (Connection conn = dataSource.getConnection(); PreparedStatement ps = conn.prepareStatement(sql)) {

            ResultSet rs = ps.executeQuery();

            if (!rs.next()) {
                return null;
            }

            List<Order> orders = new ArrayList<>();

            while (rs.next()) {
                orders.add(new Order(rs.getLong("order_id"),
                        rs.getString("ordernumber"), null));
            }

            return orders;

        } catch (SQLException e) {
            throw new RuntimeException(e);
        }
    }

顺序：

package model;

import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;

@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class Order {
    private Long id;
    private String orderNumber;
    private List<OrderRow> orderRows;
}

订单行：

package model;

import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class OrderRow {
    private String itemName;
    private Integer quantity;
    private Integer price;

}