cusId orderId itemId
00006 00003 002
00006 00003 006
00007 00001 001
00007 00001 003
00007 00004 003
00008 00002 001
00008 00002 009

002 006 -2
001 003 -1 003 -2
001 009 -2

public class Testing {
public static void main(String[] args) {
    List data = new ArrayList();
        try {
                Connection con = null;
                Class.forName("com.mysql.jdbc.Driver");
                con = DriverManager.getConnection("jdbc:mysql://localhost:3306/testing", "root", "");
                Statement st = con.createStatement();
                String query = "Select * from `order_item` as order_item left join `order` as order1 on order_item.orderId=order1.orderId order by order1.cusId, order_item.completedTime ";
                ResultSet rs = st.executeQuery(query);

                while (rs.next()) {
                    String cusId = rs.getString("cusId");
                    String orderId = rs.getString("orderId");
                    String itemId = rs.getString("itemId");
                    data.add(cusId + " " + orderId + " " + itemId);
                }

                writeToFile(data, "output.txt");
                rs.close();
                st.close();
        } catch (Exception e) {
                System.out.println(e);
        }
}

private static void writeToFile(java.util.List list, String path) {
        BufferedWriter out = null;
        try {
                File file = new File(path);
                out = new BufferedWriter(new FileWriter(file, true));
                for (Object s : list) {
                        out.write((String) s);
                        out.newLine();
                }
                out.close();
        } catch (IOException e) {
        }
}