Self Join Exercise. Have I over-complicated it? This my 3rd question on the same exercise, but by no means a duplicate.  The two previous questions were posted on StackOverflow here and here.

Now I'm posting my Oracle solution (below) that works. I wonder if the same could've been simpler and/or much more efficient. Not in terms of CTE or analytic expressions but with basic simple logic.

Data:

CREATE TABLE Readings (
  user_id varchar(10),
  reading_time int ,
  x decimal(10,2),
  y decimal(10,2)
);


INSERT ALL
    INTO Readings (user_id, reading_time, x, y) VALUES ('u1', 60, 345, 400)
    INTO Readings (user_id, reading_time, x, y) VALUES ('u1', 100, 560, 300)
    INTO Readings (user_id, reading_time, x, y) VALUES ('u2', 35, 1024, 250)
    INTO Readings (user_id, reading_time, x, y) VALUES ('u1', 90, 450, 450)
    INTO Readings (user_id, reading_time, x, y) VALUES ('u3', 150, 600, 100)
    INTO Readings (user_id, reading_time, x, y) VALUES ('u3', 100, 500, 125)
SELECT * FROM dual;


Intermediate script:

SELECT r.user_id, rm.reading_time start_time, r.reading_time end_time, 
       (r.reading_time-rm.reading_time) time_spent,
       (TO_CHAR(rm.x)||' ; '||TO_CHAR(rm.y)) start_point, 
       (TO_CHAR(r.x)||' ; '||TO_CHAR(r.y)) end_point,
       SQRT(POWER(r.x-rm.x, 2)+POWER(r.x-rm.y, 2)) distance            
FROM Readings r 
JOIN Readings rm ON (r.user_id = rm.user_id and 
                     rm.reading_time = (SELECT MAX(r2.reading_time) FROM Readings r2 WHERE r2.reading_time &lt; r.reading_time))
ORDER BY 1,2; 


Final script:

SELECT rr.user_id, SUM(rr.distance) "Total Distance", 
       SUM(rr.time_spent) "Total Time", SUM(rr.distance)/SUM(rr.time_spent) "Average Speed" 
FROM 
(SELECT r.user_id, (r.reading_time-rm.reading_time) time_spent,
        SQRT(POWER(r.x-rm.x, 2)+POWER(r.x-rm.y, 2)) distance            
FROM Readings r 
JOIN Readings rm ON (r.user_id = rm.user_id and 
                     rm.reading_time = (SELECT MAX(r2.reading_time) FROM Readings r2 WHERE r2.reading_time &lt; r.reading_time))) rr
GROUP BY rr.user_id
ORDER BY 1;


Exercise Description 

Multiple users roam a plain and at irregular time intervals report their coordinates (x, y).  This information (user id, time-stamp and the coordinates) populate table Readings. 

For each user that reported more than one set of coordinates we need to find total distance traveled, total time spent, and their average speed.

For the sake of simplicity coordinates are Cartesian and time-stamps are integers.
You have a bug. Change the row ('u3', 100, 500, 125) to ('u3', 99, 500, 125), and try again. You will see that u3 row disappears.

You can fix it with the following minimal change:

rm.reading_time = (SELECT MAX(r2.reading_time) 
  FROM Readings r2 
  WHERE  r2.user_id=r.user_id -- you have to join on ids first
  and r2.reading_time &lt; r.reading_time)

There is another bug here:
r.x-rm.y should have been r.y-rm.y.
user_id, reading_time is a unique key by problem definition. Having it declared will enable us to detect some invalid inputs. What is the average speed of an object that can be at more than one place?
I would go along with something like this for the intermediate script:

SELECT t.*, t2.*
FROM (SELECT r.user_id,r.reading_time, 
         max(r.x) AS x1, max(r.y) AS y1, 
         max(rm.reading_time) AS prev
FROM Readings r
JOIN Readings rm ON (r.user_id = rm.user_id AND rm.reading_time &lt; r.reading_time)
GROUP BY r.user_id,r.reading_time) t
INNER JOIN Readings t2
ON t.user_id = t2.user_id AND t2.reading_time = t.prev;