1. What riders belong to the team ‘Os Velozes’? (name) 
    SELECT name
FROM rider
WHERE team = 'Os Velozes'
  2. What rider won the ‘Porto - Aveiro’ stage? (name) 
    SELECT name
FROM rider JOIN
  classification USING (ref) JOIN
  stage USING (num)
WHERE description = 'Porto - Aveiro' AND
   position = 1
  3. What riders ciclistas raced the ‘Coimbra - Lisboa’ stage and what was their final position? Order the answer by their position. (name, position) 
    SELECT name, position
FROM rider JOIN
  classification USING (ref) JOIN
  stage USING (num)
WHERE description = 'Coimbra - Lisboa'
ORDER BY position
  4. How many riders are there in each team? (team, total) 
    SELECT team, COUNT(*)
FROM rider
GROUP BY team
  5. What is the total sum of times of each rider? (name, total) 
    SELECT name, SUM(time)
FROM rider JOIN
  classification USING (ref)
GROUP BY ref
  6. What team, or teams, has a smaller sum of its riders total times? (team) 
    SELECT team, SUM(time) as total
FROM rider JOIN
  classification USING (ref)
GROUP BY team
HAVING SUM(time) = (
 select min(sum_time)
 from 
 (
     SELECT SUM(time) as sum_time
     FROM rider JOIN
       classification USING (ref)
     GROUP BY team
 )
);
  7. What is the average time in each stage? (description, average) 
    SELECT description, AVG(time)
FROM stage JOIN
  classification USING (num)
GROUP BY num
  8. What stage, or stages, had the smaller average time? (description) 
    SELECT description
FROM stage JOIN
  classification USING (num)
GROUP BY num
HAVING AVG(time) = (
 select min(avg_time)
 from 
 (
     SELECT AVG(time) as avg_time
     FROM stage JOIN
          classification USING (num)
     GROUP BY num
 )
);
  9. What was the time difference between the first and second rider in each stage? (description, difference) 
    SELECT first.description, strftime('%s', second.time) - strftime('%s', first.time) as difference 
FROM
 (SELECT *
  FROM stage JOIN
    classification USING (num)
  WHERE position = 1) AS first
  JOIN
 (SELECT *
  FROM stage JOIN
    classification USING (num)
  WHERE position = 2) AS second
  USING (num);
  10. What stage had the biggest time difference between the first and second rider to finish it, what rider won that stage and with how much lead time. (description, name, difference). 
    SELECT first.description, first.name, strftime('%s', second.time) - strftime('%s', first.time) as difference 
FROM
(
    SELECT *
    FROM stage JOIN
    classification USING (num)
    join rider USING (ref)
    WHERE position = 1
) AS first
JOIN
(
    SELECT *
    FROM stage JOIN
    classification USING (num)
    WHERE position = 2
) as second 
on first.num = second.num
WHERE strftime('%s', second.time) - strftime('%s', first.time) = 
(
    select max(difference)
    from
    (
        SELECT strftime('%s', second.time) - strftime('%s', first.time) as difference
        FROM classification AS first JOIN
        classification AS second USING (num)
        WHERE first.position = 1 AND second.position = 2
    )
);

(Credits: André Restivo https://web.fe.up.pt/~arestivo)