Select
   RDB$RELATION_NAME as TABELA_SEM_PK
from RDB$RELATIONS
where
   coalesce(RDB$RELATION_TYPE,0) in ( 0, 4, 5 ) and
   ( RDB$SYSTEM_FLAG = 0 or RDB$SYSTEM_FLAG is null ) and

   RDB$RELATION_NAME NOT IN (
      Select RDB$RELATION_NAME from RDB$RELATION_CONSTRAINTS where RDB$CONSTRAINT_TYPE = 'PRIMARY KEY'
   )
order by RDB$RELATION_NAME