SQL> show version;
ISQL Version: WI-T6.0.0.1093 Firebird 6.0 9a5e091
Server version:
Firebird/Windows/AMD/Intel/x64 (access method), version "WI-T6.0.0.1093 Firebird 6.0 9a5e091"
Firebird/Windows/AMD/Intel/x64 (remote server), version "WI-T6.0.0.1093 Firebird 6.0 9a5e091/tcp (DESKTOP-MCH5AOF)/P20:C"
Firebird/Windows/AMD/Intel/x64 (remote interface), version "WI-T6.0.0.1093 Firebird 6.0 9a5e091/tcp (DESKTOP-MCH5AOF)/P20:C"
on disk structure version 14.0

with recursive
 t(n) as (
 select 0 from rdb$database
 union all
 select n + 1 from t where n < 9
 ),
 t2(n) as (
 select 100000 * t1.n + 10000 * t2.n + 1000 * t3.n + 100 * t4.n + 10 * t5.n + t6.n
 from t t1, t t2, t t3, t t4, t t5, t t6
 )
select sum(n), count(*) from t2;

 SUM COUNT
============================================= =====================
 499999500000 1000000

with
 t(n) as (
 select n from unlist('0,1,2,3,4,5,6,7,8,9' returning int) as u(n)
 ),
 t2(n) as (
 select 100000 * t1.n + 10000 * t2.n + 1000 * t3.n + 100 * t4.n + 10 * t5.n + t6.n
 from t t1, t t2, t t3, t t4, t t5, t t6
 )
select sum(n), count(*) from t2;

 SUM COUNT
============================================= =====================
 <null> 1000000

with
 t(n) as (
 select n from unlist('0,1,2,3,4,5,6,7,8,9' returning int) as u(n)
 )
select sum(100000 * t1.n + 10000 * t2.n + 1000 * t3.n + 100 * t4.n + 10 * t5.n + t6.n), count(*)
from t t1, t t2, t t3, t t4, t t5, t t6;

 SUM COUNT
============================================= =====================
 499999500000 1000000