Emular incremento automático no MySQL/InnoDB

drop table if exists users_seq;

create table users_seq
(
next_seq_id int unsigned not null default 0
)engine = innodb;

drop table if exists users;

create table users
(
user_id int unsigned not null primary key,
username varchar(32) not null
)engine = innodb;

insert into users_seq values (0);

delimiter #

create trigger users_before_ins_trig before insert on users
for each row
begin

declare id int unsigned default 0;

  select next_seq_id + 1 into id from users_seq;

  set new.user_id = id;

  update users_seq set next_seq_id = id;

end#

delimiter ;

insert into users (username) values ('f00'),('bar'),('bish'),('bash'),('bosh');

select * from users;
select * from users_seq;

insert into users (username) values ('newbie');

select * from users;
select * from users_seq;