Use CONNECT or CREATE DATABASE to specify a database
SQL> connect 'localhost/3051:test' user 'SYSDBA' password 'masterkey';
Database:  'localhost/3051:test', User: SYSDBA
SQL> SET SQL DIALECT 3;
SQL> with t(a) as (
CON>   select CAST(9999 AS NUMERIC(4, 0)) 
CON>   from rdb$database
CON>   union all
CON>   select CAST(9999 AS NUMERIC(4, 0))
CON>   from rdb$database
CON>   union all
CON>   select CAST(9999 AS NUMERIC(4, 0))
CON>   from rdb$database
CON>   union all
CON>   select CAST(9999 AS NUMERIC(4, 0))
CON>   from rdb$database
CON>   union all
CON>   select CAST(9999 AS NUMERIC(4, 0))
CON>   from rdb$database
CON>   union all
CON>   select CAST(9999 AS NUMERIC(4, 0))
CON>   from rdb$database
CON>   union all
CON>   select CAST(9999 AS NUMERIC(4, 0))
CON>   from rdb$database
CON> )
CON> select sum(a)
CON> from t;

                  SUM
=====================
                69993

SQL>