select RDB$FIELD_LENGTH, RDB$FIELD_NAME
  from RDB$FIELDS
 where not RDB$FIELD_NAME is null
-- comment
   and RDB$FIELD_LENGTH > 2
   and cast( 1 as integer ) = :PARAM_VALUE
 order by RDB$FIELD_LENGTH

create or alter procedure DEBUG
returns (
    LEN  integer,
    NAME varchar(100) )
as
  declare variable SQL blob;
begin
  SQL =
'select RDB$FIELD_LENGTH, RDB$FIELD_NAME
  from RDB$FIELDS
 where not RDB$FIELD_NAME is null
-- comment
   and RDB$FIELD_LENGTH > 2
   and cast( 1 as integer ) = :PARAM_VALUE
 order by RDB$FIELD_LENGTH
';

  for
    execute statement ( :SQL )
    ( PARAM_VALUE := 1 )
    into :LEN, :NAME
  do
    suspend;
end

Unsuccessful execution caused by system error that does not preclude successful execution of subsequent statements.
Execute statement preprocess SQL error.
At procedure 'DEBUG' line: 18, col: 3.

/*-- comment*/

create or alter procedure DEBUG
returns (
    LEN  integer,
    NAME varchar(100) )
as
  declare variable SQL blob;
begin
  SQL =
'select RDB$FIELD_LENGTH, RDB$FIELD_NAME
  from RDB$FIELDS
 where not RDB$FIELD_NAME is null
-- comment
   and RDB$FIELD_LENGTH > 2
 order by RDB$FIELD_LENGTH
';

  for
    execute statement ( :SQL )
    into :LEN, :NAME
  do
    suspend;
end

1)Если вы желаете использовать параметризированный запрос, то Вы обязаны заключить строку или переменную, содержащую текст Вашего параметризированного запроса в круглые скобки. Например:

  EXECUTE STATEMENT (:P_SQL_STMT) (P1 := 'abc', P2 := :MY_VARIABLE);