[firebird-support] how to union a dubious query to another query

[ibmcom2011]

i want to union a query A to an other query B, if no any record in a, then 
automatically switch the 3rd query C to union the B. otherwise, the A can union 
to B.

now i try to do it such as:

select 0, 0, 0 from t1       /* C */

where not exists(
  select f1, f2, f3 from t1 
        where f1 = 'xxx'     /* A */
 )

union 

select f3, f4, f5 from t2    /* B */