LISTAGG is nicely versative. (For small tables, I've gone further so as to generate INSERT statements for another DBMS when it didn't understand Oracle dates and timestamps).

SQL> create table xx ( a varchar2(5), b number, c date, d timestamp );

Table created.

SQL> insert into xx values ( 'Arby', 5, sysdate, systimestamp );

1 row created.

select 'select '
     ||listagg(case when data_type like '%CHAR%'
                    then '''"''||'||column_name||'||''"'''
                    when data_type = 'DATE'
                    then 'TO_CHAR('||column_name||',''yyyy-mm-dd hh24:mi:ss'')'
                    when substr(data_type,1,4) = 'TIME'
                    then 'TO_CHAR('||column_name||',''yyyy-mm-dd hh24:mi:ss.ff'')'
                    else column_name
                end
          ,'||'',''||')within group(order by column_id)
     ||' from xx' as builder
  from user_tab_columns
 where table_name = 'XX';

SQL> /

BUILDER
-------------------------------------------------------------------------------------------------------------------------------
select '"'||A||'"'||','||B||','||TO_CHAR(C,'yyyy-mm-dd hh24:mi:ss')||','||TO_CHAR(D,'yyyy-mm-dd hh24:mi:ss.ff') from xx

SQL> select '"'||A||'"'||','||B||','||TO_CHAR(C,'yyyy-mm-dd hh24:mi:ss')||','||TO_CHAR(D,'yyyy-mm-dd hh24:mi:ss.ff') from xx;

'"'||A||'"'||','||B||','||TO_CHAR(C,'YYYY-MM-DDHH24:MI:SS')||','||TO_CHAR(D,'YYYY-MM-DDHH24:MI:SS.
--------------------------------------------------------------------------------------------------
"Arby",5,2017-01-31 13:51:19,2017-01-31 13:51:19.673299

If you wanted to write a function to do this generically, you could use a combination of dbms_sql to generate and run the dynamic query and pipelined functions to return the result.

Hello Connor,

thanks for your reply, but I want to concat the values of every row.

In detail:
I have two tables with the 'same' data content and want to compare them to find differences.
My idea was something like this:

select table1.keyfield
from   table1, table2
where  table1.keyfield   = table2.keyfield  
  and  concat(table1.*) <> concat(table2.*)

The result should only show the rows of table1 which differs from table2 in one ore more fieldvalues.

That's the reason why I want to concat the values, not the column_names.

....
I have two tables with the 'same' data content and want to compare them to find differences
....

The FULL OUTER JOINS would be the way to go.

demo@ORA12C> create table dept1 as select * from dept;

Table created.

demo@ORA12C> create table dept2 as select * from dept;

Table created.

demo@ORA12C> update dept2 set dname = lower(dname)
  2  where rownum = 1;

1 row updated.

demo@ORA12C> commit;

Commit complete.

demo@ORA12C> select * from dept1;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON

4 rows selected.

demo@ORA12C> select * from dept2;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 accounting     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON

4 rows selected.

demo@ORA12C> select *
  2  from dept1 d1 full outer join dept2 d2
  3  on ( d1.deptno = d2.deptno and
  4   d1.dname = d2.dname and
  5   d1.loc = d2.loc )
  6  where d1.rowid is null or d2.rowid is null ;

    DEPTNO DNAME          LOC               DEPTNO DNAME          LOC
---------- -------------- ------------- ---------- -------------- -------------
                                                10 accounting     NEW YORK
        10 ACCOUNTING     NEW YORK

2 rows selected.

demo@ORA12C>

Assuming there's a primary key, I prefer the FULL OUTER coupled with DECODE in order to find the detailed discrepancies:

 select coalesce(d1.deptno,d2.deptno) as deptno
      , DECODE(d1.dname,d2.dname,'=',d1.dname||'<>'||d2.dname) as Dname
      , DECODE(d1.loc  ,d2.loc  ,'=',d1.loc  ||'<>'||d2.loc) as Dloc
   from dept1 d1
   full outer
   join dept2 d2
     on d1.deptno = d2.deptno
 order by 1, 2
 /

DEPTNO DNAME                          DLOC
------ ------------------------------ ----------------------------
    10 ACCOUNTING<>accounting         =
    20 =                              =
    30 =                              =
    40 =                              =

Another way would be to use UNION ALL with group by.

Check the following link on how to
https://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:9531089900346576427#9531095900346064037