come creare un trigger di aggiornamento per aumentare/diminuire 1 numero al numero totale di voti

Sì, devi creare un after insert trigger per quello

delimiter //
create trigger total_votes_count after insert on votes
for each row
begin
 if (new.value == 1) then
   update posts set total_votes = total_votes+1 
   where id = new.id_post;
 elseif (new.value == -1) then
   update posts set total_votes = total_votes-1 
   where id = new.id_post;
 end if;
end;//

delimiter //

Per gestire l'aggiornamento tutto rimane lo stesso, solo tu hai bisogno di un altro trigger qualcosa come

delimiter //
    create trigger total_votes_count_upd after update on votes
    for each row
    begin
     if (new.value == 1) then
       update posts set total_votes = total_votes+1 
       where id = new.id_post;
     elseif (new.value == -1) then
       update posts set total_votes = total_votes-1 
       where id = new.id_post;
     end if;
    end;//

    delimiter //

Dato che hai 2 tabelle di post, dovrai usarle nella condizione if

delimiter //
create trigger total_votes_count after insert on votes
for each row
begin
 if (new.value == 1) then
   if (new.table_name == 'post_A') then 
     update posts_A set total_votes = total_votes+1 
     where id = new.id_post;
   else
     update posts_B set total_votes = total_votes+1 
     where id = new.id_post;
   end if;
 elseif (new.value == -1) then
   if (new.table_name == 'post_A') then
      update posts_A set total_votes = total_votes-1 
      where id = new.id_post;
   else
      update posts_B set total_votes = total_votes-1 
      where id = new.id_post;
   end if ; 
 end if;
end;//

delimiter //

Fai lo stesso per il trigger di aggiornamento.