CREATE TABLE your_table ( your_list_column ) AS
  SELECT 'a,a,b,c,d' FROM DUAL UNION ALL -- duplicates both at head
  SELECT 'a,d' FROM DUAL UNION ALL -- duplicates at head and middle
  SELECT 'a,d,a' FROM DUAL UNION ALL -- duplicates at head and tail
  SELECT 'a,d' FROM DUAL UNION ALL -- duplicates at middle and next item
  SELECT 'a,d' FROM DUAL UNION ALL -- duplicates at middle and middle
  SELECT 'a,b' FROM DUAL UNION ALL -- duplicates at middle and tail
  SELECT 'a,d' FROM DUAL UNION ALL -- duplicates both at tail
  SELECT 'a,b' FROM DUAL UNION ALL -- two pairs of duplicates
  SELECT 'a,e' FROM DUAL;          -- no duplicates

要获取具有重复值的列表,可以在正则表达式中使用反向引用：

SELECT *
FROM   your_table
WHERE  REGEXP_LIKE( ',' || your_list_column || ',',([^,]+),(.+,)?\1,' )

输出：

YOUR_LIST_COLUMN
----------------
a,d
a,a
a,b
a,b

要获得第一个重复值,您可以提取上述正则表达式的第一个子组：

SELECT your_list_column,REGEXP_SUBSTR( ',1,NULL,1 )
         AS duplicate_value
FROM   your_table
WHERE  REGEXP_LIKE( ',' )

输出：

YOUR_LIST_COLUMN DUPLICATE VALUE
---------------- ---------------
a,d        a
a,a        a
a,d        b
a,b        b
a,d        d
a,b        a

要获得唯一值,请使用split_string() function as defined here(但使用用户定义的类型而不是预定义的VARRAY)：

CREATE OR REPLACE TYPE stringlist IS TABLE OF VARCHAR2(4000);
/

CREATE OR REPLACE FUNCTION split_String(
  i_str    IN  VARCHAR2,i_delim  IN  VARCHAR2 DEFAULT ','
) RETURN stringlist DETERMINISTIC
AS
  p_result       stringlist := stringlist();
  p_start        NUMBER(5) := 1;
  p_end          NUMBER(5);
  c_len CONSTANT NUMBER(5) := LENGTH( i_str );
  c_ld  CONSTANT NUMBER(5) := LENGTH( i_delim );
BEGIN
  IF c_len > 0 THEN
    p_end := INSTR( i_str,i_delim,p_start );
    WHILE p_end > 0 LOOP
      p_result.EXTEND;
      p_result( p_result.COUNT ) := SUBSTR( i_str,p_start,p_end - p_start );
      p_start := p_end + c_ld;
      p_end := INSTR( i_str,p_start );
    END LOOP;
    IF p_start <= c_len + 1 THEN
      p_result.EXTEND;
      p_result( p_result.COUNT ) := SUBSTR( i_str,c_len - p_start + 1 );
    END IF;
  END IF;
  RETURN p_result;
END;
/

然后你可以将它与SET()集合函数结合使用：

SELECT t.*,(
         SELECT LISTAGG( COLUMN_VALUE,' ) WITHIN GROUP ( ORDER BY ROWNUM )
         FROM   TABLE( SET( split_string( t.your_list_column ) ) )
       ) AS unique_list
FROM   your_table t

输出：

YOUR_LIST_COLUMN UNIQUE_LIST
---------------- ---------------
a,d        a,a        a,b        a,c
a,e        a,e