SELECT t1.num, t2.val FROM qw AS t1, qw AS t2  WHERE t1.num <= t2.num;

/* CTE
WITH  qw AS( 
SELECT 1 AS num, 'a' AS val UNION ALL
SELECT 2,'b' UNION ALL
SELECT 3,'c' UNION ALL
SELECT 4,'d') */
SELECT t1.num, t2.val 
FROM 
qw AS t1, 
(SELECT RAND()*maxnum.k +1 AS kn, val 
FROM qw, (SELECT MAX(num) AS k FROM qw) AS maxnum) AS t2
  WHERE  t1.num <=t2.kn;
-- 
-- (SELECT MAX(num) AS k FROM qw) as maxnum  -- подзапрос для вычисления МАХ num = maxnum.k
-- RAND()*maxnum.k +1 - генерим Kn , важна только целая часть, с ROUND делайте сами, если надо