Extract Vowels from a given string

SQL> SELECT LISTAGG (vowels_temp, ',') WITHIN GROUP (ORDER BY pos) AS vowels
  2    FROM (SELECT pos,
  3                 CASE
  4                    WHEN alpha IN ('a',
  5                                   'e',
  6                                   'i',
  7                                   'o',
  8                                   'u')
  9                    THEN
 10                       alpha
 11                    ELSE
 12                       NULL
 13                 END
 14                    AS vowels_temp
 15            FROM (    SELECT LEVEL AS pos, SUBSTR ('rajasekhar', LEVEL, 1) AS alpha
 16                        FROM DUAL
 17                  CONNECT BY LEVEL <= LENGTH ('rajasekhar')))
 18   WHERE vowels_temp IS NOT NULL;

VOWELS
--------------------------------------------------------------------------------
a,a,e,a

SQL>