Potresti usare una tabella e dei trigger per implementare una sequenza simile a un oracolo:
drop table if exists users_seq;
create table users_seq
(
next_val int unsigned not null default 0
)
engine=innodb;
drop table if exists users;
create table users
(
user_id int unsigned not null,
username varchar(32) unique not null
)
engine=innodb;
delimiter #
create trigger users_before_ins_trig before insert on users
for each row
begin
declare v_id int unsigned default 0;
select next_val + 1 into v_id from users_seq;
set new.user_id = v_id;
update users_seq set next_val = v_id;
end#
delimiter ;
insert into users_seq values (0);
insert into users (username) values ('alpha'),('beta');
Query OK, 2 rows affected, 1 warning (0.03 sec)
select * from users_seq;
+----------+
| next_val |
+----------+
| 2 |
+----------+
1 row in set (0.00 sec)
select * from users;
+---------+----------+
| user_id | username |
+---------+----------+
| 1 | alpha |
| 2 | beta |
+---------+----------+
2 rows in set (0.00 sec)
insert into users (username) values ('alpha');
ERROR 1062 (23000): Duplicate entry 'alpha' for key 'username'
select * from users_seq;
+----------+
| next_val |
+----------+
| 2 |
+----------+
1 row in set (0.00 sec)
select * from users;
+---------+----------+
| user_id | username |
+---------+----------+
| 1 | alpha |
| 2 | beta |
+---------+----------+
2 rows in set (0.00 sec)
insert into users (username) values ('gamma');
Query OK, 1 row affected, 1 warning (0.03 sec)
select * from users_seq;
+----------+
| next_val |
+----------+
| 3 |
+----------+
1 row in set (0.00 sec)
select * from users;
+---------+----------+
| user_id | username |
+---------+----------+
| 1 | alpha |
| 2 | beta |
| 3 | gamma |
+---------+----------+
3 rows in set (0.00 sec)
spero che ti aiuti :)
