create table t1 (n integer);

insert into t1 values (1);
insert into t1 values (2);
insert into t1 values (3);

-- ok
SQL> select a.*
CON> from t1 a
CON> where a.rdb$db_key = (
CON> select b.rdb$db_key
CON> from t1 b
CON> where n = 1
CON> );

           N
============
           1

-- ok
SQL> create view v1 as
CON> select a.*
CON> from t1 a
CON> where a.rdb$db_key = (
CON> select b.rdb$db_key
CON> from t1 b
CON> where n = 1
CON> );
SQL>

-- ok
SQL> select * from v1;

           N
============
           1

-- wrong: returns nothing
SQL> select * from v1 union all select * from v1;