Question

I’m trying to generate a sequence table in MySQL, so that I can get unique ids from last_insert_id.

The problem is that I need multiple sequences dynamically.

---

At the first, I created a table:

CREATE TABLE `sequence` (
  `label` char(30) CHARACTER SET latin1 NOT NULL,
  `id` mediumint(9) NOT NULL DEFAULT '0',
  PRIMARY KEY (`label`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

And then tried to get the number, using example from http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_last-insert-id

UPDATE sequence SET id = LAST_INSERT_ID(id + 1) WHERE label = 'test';
SELECT LAST_INSERT_ID();

After a while I realized that I also need to generate rows for new labels safely.
So I changed this schema into:

CREATE TABLE `sequence` (
  `label` char(30) CHARACTER SET latin1 NOT NULL,
  `id` mediumint(9) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`label`,`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8

And I simply gave up using WHERE clause to update its id.

INSERT INTO sequence (label) values ( ? )
SELECT LAST_INSERT_ID()

Is this a proper way? I want to know if there is a better solution.

Answer 1

The MyISAM engine will do it for you –

Table definition:

CREATE TABLE `sequence` (
  `label` char(30) CHARACTER SET latin1 NOT NULL,
  `id` mediumint(9) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`label`,`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

Populate table:

INSERT INTO sequence VALUES ('a', NULL); -- add some 'a' labels
INSERT INTO sequence VALUES ('a', NULL);
INSERT INTO sequence VALUES ('a', NULL);

INSERT INTO sequence VALUES ('b', NULL); -- add another labels 'b'
INSERT INTO sequence VALUES ('b', NULL);

INSERT INTO sequence VALUES ('a', NULL); -- add some 'a' labels
INSERT INTO sequence VALUES ('a', NULL);

Show result:

SELECT * FROM sequence;
+-------+----+
| label | id |
+-------+----+
| a     |  1 |
| a     |  2 |
| a     |  3 |
| a     |  4 |
| a     |  5 |
| a     |  6 |
| b     |  1 |
| b     |  2 |
+-------+----+