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.)