Return default data if no data is returned by sql query

Question

Hi,
I have a requirement.
I have an inner query that returns no data. However I still want to return default data (say 'abc'). The below sql does not return anything inspite of using nvl . Please advise .
select nvl( x.vencode,'abc') vencode
from
-- below sql returns no data!
          SELECT a.vencode vencode,
                 a.mid mid,
                 a.title title,
                  ROW_NUMBER() OVER (PARTITION BY a.vencode ORDER BY a.title) rnk
           FROM (
                 SELECT *
                 FROM    (SELECT vencode FROM vendor where login is not null and vencode = 'BKFI'
                                    and login > (sysdate - 90) )
                      CROSS JOIN
                         (SELECT mid, title, ROWNUM num FROM tcommmemos where mid is not null)
                ) a,        
               (SELECT * FROM (
                SELECT v.vencode, tc.mid, tc.title
           FROM
           (select  * from vendor
                 where login is not null and vencode = 'BKFI'
                 and login > (sysdate - 90)) v,
            tcommmemosviewed tcv, tcommmemos tc
           WHERE     v.vencode = tcv.vencode
                 and tc.mid is not null
                 --AND tc.post_date > v.hiredate
                 AND tc.mid = tcv.mid)
               ) b
               where a.vencode = b.vencode(+)
                 and a.title   = b.title(+)
                 and a.mid     = b.mid(+)
                 and b.mid is null                               
) x              

Answer 1

Use any aggregate function it'll always return a value even if there no data in the table
create table r_dummy_1 (a number);
SQL> select * from r_dummy_1;
no rows selected
SQL> select nvl(max(a),1) from r_dummy_1;
NVL(MAX(A),1)
1