SQL> show table sec$users;
SEC$USER_NAME                   (RDB$USER) CHAR( 31 ) CHARACTER SET  UNICODE_FSS  Nullable
. . .

SQL> create or alter user "ВасяПупкинКулха" password '123';

SQL> create or alter user "ВасяПупкинКулха ц " password '123';
Statement failed, SQLSTATE = 22001
arithmetic exception, numeric overflow, or string truncation
-string right truncation
-expected length 31, actual 16

SQL> alter user "ВасисуалийПупка" firstname 'Vasisualiy';
SQL> alter user "ВасисуалийПупка" password '000';
SQL> alter user "ВасисуалийПупка" tags( city = 'Balaha', favorite_group = 'Deep Purple');

recreate table t1(id int); 
commit;
insert into t1 values( 1 ); 
commit;
select a.id from t1  a , t1  b  join t1  c  on a.id=c.id where a.id=b.id; -- этот завалится
select a.id from t1  b , t1  a  join t1  c  on a.id=c.id where a.id=b.id; -- а этот - отработает без ошибок :-)

@echo off
setlocal enabledelayedexpansion enableextensions
cls
set fq=%1
if .%fq%.==.. set fq=3
set sql=%~n0.tmp

del %sql% 2>nul
echo recreate exception ex_test 'Hi from Mariana Trench, depth=@1';>>%sql%
echo recreate sequence g;>>%sql%
echo commit;>>%sql%
echo set term ^^;>>%sql%
echo execute block as>>%sql%
echo declare n int = 0;>>%sql%
echo begin>>%sql%
for /l %%i in (1, 1, %fq%) do ( 
  echo begin -- level = %%i>>%sql%
  echo   n = gen_id(g, 1^);>>%sql%
)
for /l %%i in (%fq%, -1, 1) do (
  if .%%i.==.%fq%. echo exception ex_test using( n ^);>>%sql%
  echo end -- level = %%i>>%sql%
)

echo end^^>>%sql%
echo set term ;^^>>%sql%
echo commit;>>%sql%

execute block returns(how_much varchar(20)) as
    declare n int;
begin
    n = rand()*1000;
    if (n < 100) then how_much = 'poor';
    else  if  ( n < 300 ) then how_much = 'middle';
    else  if  ( n < 700 ) then how_much = 'optimal';
    else how_much = 'high';
    suspend;
end