SELECT table1.column1, 
       table1.column2, 
       table1.column3, 
       table1.column4, 
       SUM(CASE 
             WHEN ( 1 - table1.age4 ) <= 1 THEN 0 
             WHEN ( 1 - table1.age4 ) > 1 THEN ( ( table2.age5 ) * ( 1 / 
                                                 ( 1 - table1.age4 ) ) ) 
             ELSE 0 
           END) AS COLUMN5 
FROM   table1 
       inner join table2 
               ON table2.column1 = table1.column1 
GROUP  BY table1.column1, 
          table1.column2, 
          table1.column3, 
          table1.column4 
ORDER  BY table1.column1, 
          table1.column2, 
          table1.column3, 
          table1.column4; 

SQL> select case when 1-1 <= 1 then 0
  2              when 1-1 > 1 then 2 * ( 1 / (1-1) )
  3              else 0
  4         end val
  5  from dual;
       VAL
----------
         0