SQL & PL/SQL

Hi,

I was trying to use "with" clause with union all ...but it gives me error ....

   with t1 as 
        (select '1'col1 from dual union all
         select '2' col2 from dual)select * from t1
         union 
        with t2 as 
        (select '3' col1 from dual union all
         select '4' col1 from dual )
          select * from t2
            

What wrong am i doing here ??? Thank you so much!!