Oracle - Error at line 4: PL/SQL: SQL Statement - sql

Question

I am trying to build a trigger for my database and I am getting an error and I suspect it is because of my into clause but I am not getting the reason. I was reading the documentation of it (https://docs.oracle.com/cd/B19306_01/appdev.102/b14261/selectinto_statement.htm) and there is saying:
By default, a SELECT INTO statement must return only one row.
Otherwise, PL/SQL raises the predefined exception TOO_MANY_ROWS and
the values of the variables in the INTO clause are undefined. Make
sure your WHERE clause is specific enough to only match one row
Well, at least I am trying to be sure that my where clause is returning one and just one row. Can you give me some advice about it?
CREATE OR REPLACE TRIGGER t_ticket
INSTEAD OF INSERT ON V_buyTicket FOR EACH ROW
declare ticketID number; busy number; seatRoom number;
BEGIN
select count(a.id_ticket) into busy , s.freeSeats into seatRoom
from assigned a
inner join show e on (a.id_movie= e.id_movie)
inner join rooms s on (e.id_room = s.id_room)
where a.id_session = 1 AND a.id_movie = 1
group by s.freeSeats;
if(busy < seatRoom ) then
ticketID := seq_ticket.NEXTVAL;
insert into tickets(id_ticket, type, number, cc, store) values(ticketID, :new.type, :new.number, :new.cc, :new.store);
insert into assigned (id_ticket, id_movie, id_session) values(ticketID, :new.id_movie, :new.id_session);
else
DBMS_OUTPUT.PUT_LINE('No available seats');
end if;
END;

Answer 1

Don't use group by if you want exactly one row returned:
select count(a.id_ticket), sum(s.freeSeats)
into busy, seatRoom
from assigned a inner join
show e
on a.id_movie = e.id_movie inner join
rooms s
on e.id_room = s.id_room
where a.id_session = 1 and a.id_movie = 1;
An aggregation query without a group by always returns exactly one row. The alternative would be to include an explicit rownum = 1 in the WHERE clause.
My guess is that you conditions are not choosing exactly one room. You might want to check on the logic.