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

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

           N
============
           1
           2
           3