Using AUTO_INCREMENT

The AUTO_INCREMENT attribute can be used to generate a unique identity for new rows:

CREATE TABLE animals (
             id MEDIUMINT NOT NULL AUTO_INCREMENT,
             name CHAR(30) NOT NULL,
             PRIMARY KEY (id)
             );
INSERT INTO animals (name) VALUES ("dog"),("cat"),("penguin"),
                                  ("lax"),("whale"),("ostrich");
SELECT * FROM animals;

Which returns:

+----+---------+
| id | name    |
+----+---------+
|  1 | dog     |
|  2 | cat     |
|  3 | penguin |
|  4 | lax     |
|  5 | whale   |
|  6 | ostrich |
+----+---------+

You can retrieve the most recent AUTO_INCREMENT value with the LAST_INSERT_ID() SQL function or the mysql_insert_id() C API function. Note: For a multiple-row insert, LAST_INSERT_ID()/mysql_insert_id() will actually return the AUTO_INCREMENT key from the first of the inserted rows. This allows multiple-row inserts to be reproduced correctly on other servers in a replication setup.

For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column)+1) WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.

CREATE TABLE animals (
             grp ENUM('fish','mammal','bird') NOT NULL,
             id MEDIUMINT NOT NULL AUTO_INCREMENT,
             name CHAR(30) NOT NULL,
             PRIMARY KEY (grp,id)
             );
INSERT INTO animals (grp,name) VALUES("mammal","dog"),("mammal","cat"),
                  ("bird","penguin"),("fish","lax"),("mammal","whale"),
                  ("bird","ostrich");
SELECT * FROM animals ORDER BY grp,id;

Which returns:

+--------+----+---------+
| grp    | id | name    |
+--------+----+---------+
| fish   |  1 | lax     |
| mammal |  1 | dog     |
| mammal |  2 | cat     |
| mammal |  3 | whale   |
| bird   |  1 | penguin |
| bird   |  2 | ostrich |
+--------+----+---------+

Note that in this case (when the AUTO_INCREMENT column is part of a multiple-column index), AUTO_INCREMENT values will be reused if you delete the row with the biggest AUTO_INCREMENT value in any group. This happens even for MyISAM tables, for which AUTO_INCREMENT values normally are not reused.)