这里有一个方法。

with tmp (str, new_str,iter) as (
      select '', 'abcdaeeebfghieejklca', 0 from dual
      union all
      select new_str, regexp_replace(new_str, '(.)(.*?)\1+', '\1\2'), iter + 1
      from tmp
      where str != new_str or iter = 0
    )
    select max(str) keep(dense_rank last order by iter) as str
    from tmp;
 
STR
------------
abcdefghijkl

要查找所有字符只出现一次的字符串，可以使用正则表达式(.).*?\1查找具有重复字符的行并排除它们：

SELECT value
FROM   table_name
WHERE  NOT REGEXP_LIKE(value, '(.).*?\1');

其中，对于样本数据：

CREATE TABLE table_name (value) AS
  SELECT 'a' FROM DUAL UNION ALL
  SELECT 'bb' FROM DUAL UNION ALL
  SELECT 'ccc' FROM DUAL UNION ALL
  SELECT 'd' FROM DUAL UNION ALL
  SELECT 'abcd' FROM DUAL UNION ALL
  SELECT 'abcad' FROM DUAL;

输出：
| 价值|
| --------------|
| 一种|
| d|
| 阿拉伯基督教民主联盟|
fiddle