DELIMITER //

create procedure delete_reply_record( param int )
begin
declare fk  int;
declare cnt int;

/* Получаем внешний ключ */
select distinct
    id_test_fk
into fk from
    dbschool.reply
where
    reply.id_reply_pk = param;

/* Удаляем запись */
delete from dbschool.reply where reply.id_reply_pk = param;
 
/* Получаем количество оставшихся записей */
select 
    count(*)
into cnt from
    dbschool.reply
where
    reply.id_test_fk = fk;

/*Удаляем или обновляем*/
if cnt = 0 then 
    delete from dbschool.tests where tests.id_test_pk = fk;
else
    update dbschool.tests set count_reply=cnt where id_test_pk = fk;
end if;
    
end; //