3.6.9 Using AUTO_INCREMENT
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;
+----+---------+ | 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 0 to the column to generate sequence numbers, unless the
NO_AUTO_VALUE_ON_ZERO SQL mode is enabled. For example:
INSERT INTO animals (id,name) VALUES(0,'groundhog');
If the column is declared
NOT NULL, it is also possible to assign
NULL to the column to generate sequence numbers. For example:
INSERT INTO animals (id,name) VALUES(NULL,'squirrel');
When you insert any other value into an
AUTO_INCREMENT column, the column is set to that value and the sequence is reset so that the next automatically generated value follows sequentially from the largest column value. For example:
INSERT INTO animals (id,name) VALUES(100,'rabbit'); INSERT INTO animals (id,name) VALUES(NULL,'mouse'); SELECT * FROM animals; +-----+-----------+ | id | name | +-----+-----------+ | 1 | dog | | 2 | cat | | 3 | penguin | | 4 | lax | | 5 | whale | | 6 | ostrich | | 7 | groundhog | | 8 | squirrel | | 100 | rabbit | | 101 | mouse | +-----+-----------+
Updating an existing
AUTO_INCREMENT column value in an
InnoDB table does not reset the
AUTO_INCREMENT sequence as it does for
You can retrieve the most recent automatically generated
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 the smallest integer data type for the
AUTO_INCREMENT column that is large enough 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. Use the
UNSIGNED attribute if possible to allow a greater range. For example, if you use
TINYINT, the maximum permissible sequence number is 127. For
TINYINT UNSIGNED, the maximum is 255. See Section 11.1.2, “Integer Types (Exact Value) - INTEGER, INT, SMALLINT, TINYINT, MEDIUMINT, BIGINT” for the ranges of all the integer types.
For a multiple-row insert,
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.
mysql> ALTER TABLE tbl AUTO_INCREMENT = 100;
For information about
AUTO_INCREMENT usage specific to
InnoDB, see Section 22.214.171.124, “AUTO_INCREMENT Handling in InnoDB”.
MyISAMtables, you can specify
AUTO_INCREMENTon a secondary column in a multiple-column index. In this case, the generated value for the
AUTO_INCREMENTcolumn is calculated as
MAX(. This is useful when you want to put data into ordered groups.
auto_increment_column) + 1 WHERE prefix=
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;
+--------+----+---------+ | 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_INCREMENTcolumn is part of a multiple-column index),
AUTO_INCREMENTvalues are reused if you delete the row with the biggest
AUTO_INCREMENTvalue in any group. This happens even for
MyISAMtables, for which
AUTO_INCREMENTvalues normally are not reused.
AUTO_INCREMENTcolumn is part of multiple indexes, MySQL generates sequence values using the index that begins with the
AUTO_INCREMENTcolumn, if there is one. For example, if the
animalstable contained indexes
PRIMARY KEY (grp, id)and
INDEX (id), MySQL would ignore the
PRIMARY KEYfor generating sequence values. As a result, the table would contain a single sequence, not a sequence per
More information about
AUTO_INCREMENT is available here:
AUTO_INCREMENTvalue to be used: Section 5.1.7, “Server System Variables”.
AUTO_INCREMENTand replication: Section 126.96.36.199, “Replication and AUTO_INCREMENT”.