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3.6.9. 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)
) ENGINE=MyISAM;

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 |
+----+---------+
No value was specified for the AUTO_INCREMENT column, so MySQL assigned sequence 
numbers automatically. You can also explicitly assign NULL or 0 to the column to 
generate sequence numbers.

You can retrieve the most recent AUTO_INCREMENT value with the LAST_INSERT_ID() 
SQL function or the mysql_insert_id() C API function. These functions are 
connection-specific, so their return values are not affected by another 
connection which is also performing inserts.

Use a large enough integer data type for the AUTO_INCREMENT column to hold the 
maximum sequence value you will need. When the column reaches the upper limit 
of the data type, the next attempt to generate a sequence number fails. For 
example, if you use TINYINT, the maximum permissible sequence number is 127. 
For TINYINT UNSIGNED, the maximum is 255.

Note
For a multiple-row insert, LAST_INSERT_ID() and mysql_insert_id() actually 
return the AUTO_INCREMENT key from the first of the inserted rows. This enables 
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)
) ENGINE=MyISAM;

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 |
+--------+----+---------+
In this case (when the AUTO_INCREMENT column is part of a multiple-column 
index), AUTO_INCREMENT values are 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.

If the AUTO_INCREMENT column is part of multiple indexes, MySQL will generate 
sequence values using the index that begins with the AUTO_INCREMENT column, if 
there is one. For example, if the animals table contained indexes PRIMARY KEY 
(grp, id) and INDEX (id), MySQL would ignore the PRIMARY KEY for generating 
sequence values. As a result, the table would contain a single sequence, not a 
sequence per grp value.

To start with an AUTO_INCREMENT value other than 1, you can set that value with 
CREATE TABLE or ALTER TABLE, like this:

mysql> ALTER TABLE tbl AUTO_INCREMENT = 100;
More information about AUTO_INCREMENT is available here:

How to assign the AUTO_INCREMENT attribute to a column: Section 12.1.10, 
“CREATE TABLE Syntax”, and Section 12.1.4, “ALTER TABLE Syntax”.

How AUTO_INCREMENT behaves depending on the NO_AUTO_VALUE_ON_ZERO SQL mode: 
Section 5.1.6, “Server SQL Modes”.

How to use the LAST_INSERT_ID() function to find the row that contains the 
most recent AUTO_INCREMENT value: Section 11.13, “Information Functions”.

Setting the AUTO_INCREMENT value to be used: Section 5.1.3, “Server System 
Variables”.

AUTO_INCREMENT and replication: Section 15.4.1.1, “Replication and 
AUTO_INCREMENT”.

Server-system variables related to AUTO_INCREMENT (auto_increment_increment 
and auto_increment_offset) that can be used for replication: Section 5.1.3, 
“Server System Variables”.

Snippet from the MySQL Docs Online.