SQL> COLUMN converted FORMAT A40
SQL> SELECT   MAX(DECODE(pivot.rn,1,pivot.my_column))
  2  ||       MAX(DECODE(pivot.rn,2,',' || pivot.my_column))
  3  ||       MAX(DECODE(pivot.rn,3,',' || pivot.my_column))
  4  ||       MAX(DECODE(pivot.rn,4,',' || pivot.my_column))
  5  ||       MAX(DECODE(pivot.rn,5,',' || pivot.my_column))
  6  ||       MAX(DECODE(pivot.rn,6,',' || pivot.my_column))
  7  ||       MAX(DECODE(pivot.rn,7,',' || pivot.my_column))
  8  ||       MAX(DECODE(pivot.rn,8,',' || pivot.my_column)) converted
  9  FROM    (SELECT   ord_tbl.my_column
 10           ,        ROWNUM   rn
 11           FROM    (SELECT   my_column
 12                    FROM     my_table
 13                    ORDER BY my_column) ord_tbl) pivot
 14  /
  
CONVERTED
----------------------------------------
dd,df,dfg,fdg,we

SQL> CREATE OR REPLACE FUNCTION to_row
  2  RETURN VARCHAR2
  3  IS
  4      CURSOR c_columns IS
  5          SELECT   mt.my_column
  6          FROM     my_table  mt
  7          ORDER BY mt.my_column;
  8      
  9      l_string        VARCHAR2(4000);
 10  BEGIN   
 11      FOR sep_by_commas IN c_columns LOOP
 12          l_string := l_string
 13                      || sep_by_commas.my_column
 14                      || ',';
 15      END LOOP;
 16      RETURN(SUBSTR(l_string,1,LENGTH(l_string) - 1));    
 17  END to_row;
 18  /
  
Function created.
  
SQL> SELECT to_row AS converted FROM DUAL;
  
CONVERTED
----------------------------------------
dd,df,dfg,fdg,we
  
SQL>