Firebird Stored Procedure: Select first seat available in list

SET TERM !! ;
create procedure reserve_seat(new_reservation_id integer)
returns (seat_id integer)
as
  DECLARE VARIABLE num_free_seats INTEGER;
  DECLARE VARIABLE free_seat_id INTEGER;
  DECLARE VARIABLE reserved_seat_id INTEGER;
begin
  free_seat_id = -1;
  num_free_seats = -1;
  reserved_seat_id = -1;
  while ((num_free_seats < 0) or ((num_free_seats > 0) and (reserved_seat_id <= 0))) do
  begin
  
    select count(*) 
    from seats
    where (taken = 0 or taken is null)
    into :num_free_seats;
    
    if (num_free_seats is null) then
      num_free_seats = 0;
      
    if (num_free_seats > 0) then
    begin
      select seat_id 
      from seats
      where ((taken = 0) or (taken is null))
        and ((reservation_id) = 0 or (reservation_id is null))
      into :free_seat_id;
 
      if (free_seat_id is null) then
        free_seat_id = 0;
 
      if (free_seat_id > 0) then
      begin
        update seats
        set taken = 1, reservation_id = :new_reservation_id
        where ((taken = 0) or (taken is null))
          and ((reservation_id) = 0 or (reservation_id is null))
          and (seat_id = :free_seat_id);
          
        select seat_id
        from seats
        where (taken = 1)
          and (reservation_id = :new_reservation_id)
          and (seat_id = :free_seat_id)
        into :reserved_seat_id;
        
        if (reserved_seat_id is null) then
          reserved_seat_id = 0;
      end
    end
  end
end !!
SET TERM; !!

                                              

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CREATE TABLE dummy2
(
    "id" numeric(18,0) not null primary key,
    "dummytext" varchar(1024)
);
                                              

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Explanation of code:
This procedure takes two input parameters: DaBookingID, and DaFlightID, and returns two output pramaters: DaSeatNumber, and Successful?
 
 SET TERM ^ ;
 
CREATE OR ALTER PROCEDURE "reserve_seat" (
    "DaBookingID" integer,
    "DaFlightID" integer)
returns (
    "DaSeatNumber" varchar(5),
    "Successful" char(1))
as
declare variable "DaSeatID" integer;
declare variable "num_free_seats" integer;
begin
  "DaSeatNumber" = null;
  "DaSeatID" = -1;
  "Successful" = 'T';
  "num_free_seats" = -1;
 */Check if there are in fact any free seats for a FlightID, based on if they have a passengerID or not/*
    select count(*)
    from "TblFlightScheduleSeats"
    where ("PassengerID" is null AND "FlightID" = :"DaFlightID")
    into "num_free_seats";
 
    if ("num_free_seats" is null) then
      "num_free_seats" = 0;
 
    if ("num_free_seats" = 0) then
    begin
      "Successful" = 'F';
    end
 
    if ("num_free_seats" > 0) then
    begin
      SELECT FIRST 1 "ID" FROM "TblFlightScheduleSeats" WHERE ("PassengerID" is null AND "FlightID" = :"DaFlightID") INTO :"DaSeatID";
      SELECT FIRST 1 "SeatNumber" FROM "TblFlightScheduleSeats" WHERE ("PassengerID" is null AND "FlightID" = :"DaFlightID") INTO :"DaSeatNumber";
 
      UPDATE "TblFlightScheduleSeats" SET "PassengerID" = :"DaBookingID" WHERE "ID" = :"DaSeatID";
    end
    SUSPEND;
 end^
 
SET TERM ; ^
 
GRANT SELECT,UPDATE ON "TblFlightScheduleSeats" TO PROCEDURE "reserve_seat";
 
GRANT EXECUTE ON PROCEDURE "reserve_seat" TO SYSDBA;

                                              

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