Automatically generate unique identities for new rows. This attribute is commonly used for primary keys to ensure sequential numbering.
The AUTO_INCREMENT attribute can be used to generate a unique identity for new rows. When you insert a new record into the table (or upon adding an AUTO_INCREMENT attribute with the ALTER TABLE statement), and the AUTO_INCREMENT field is NULL or DEFAULT (in the case of an INSERT), automatically be incremented. This also applies to 0, unless the NO_AUTO_VALUE_ON_ZERO SQL_MODE is enabled.
AUTO_INCREMENT columns start from 1 by default. The automatically generated value can never be lower than 0.
Each table can have only one AUTO_INCREMENT column. It must defined as a key (not necessarily the PRIMARY KEY or UNIQUE key). In some storage engines (including the default ), if the key consists of multiple columns, the AUTO_INCREMENT column must be the first column. Storage engines that permit the column to be placed elsewhere are , , , , , , and .
SERIAL is an alias for BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE.
You can use an statement to assign a new value to the auto_increment table option, or set the server system variable to change the next AUTO_INCREMENT value inserted by the current session.
can be used to see the last AUTO_INCREMENT value inserted by the current session.
AUTO_INCREMENT is persistent in InnoDB.
See also .
It is possible to specify a value for an AUTO_INCREMENT column. If the key is primary or unique, the value must not already exist in the key.
If the new value is higher than the current maximum value, the AUTO_INCREMENT value is updated, so the next value will be higher. If the new value is lower than the current maximum value, the AUTO_INCREMENT value remains unchanged.
The following example demonstrates these behaviors:
The storage engine does not allow to insert a value that is lower than the current maximum.
An AUTO_INCREMENT column normally has missing values. This happens because if a row is deleted, or an AUTO_INCREMENT value is explicitly updated, old values are never re-used. The REPLACE statement also deletes a row, and its value is wasted. With InnoDB, values can be reserved by a transaction; but if the transaction fails (for example, because of a ROLLBACK) the reserved value will be lost.
Thus AUTO_INCREMENT values can be used to sort results in a chronological order, but not to create a numeric sequence.
To make master-master or Galera safe to use AUTO_INCREMENT , you should use the system variables and to generate unique values for each server.
If auto_increment_offset is larger than auto_increment_increment, the value of auto_increment_offset is ignored, and the offset reverts to the default of 1 instead:
auto_increment_increment and auto_incremenet_offset when adding a new master to a multi-master setupThe purpose of and is to ensure that in a multi-master or multi-source setup, all masters generate unique values for auto_increment fields or for sequences with INCREMENT=0.
If auto_increment_increment is larger than the current number of masters, then one can configure the new master with the not used auto_increment_offset. The easiest way to add a new master is to stop all MariaDB servers, update auto_increment_increment and auto_increment_offset in the configuration files, and restart.\
This has to be done if auto_increment_increment is 1. If one has more than one master (auto_increment_increment > 1), there is a way to add more masters with only having to restart one of the masters. The 'trick' is to configure one of the masters to not use all the values in its current sequence.
The following example illustrates how to do it. Assume you have two masters, A and B.
In this case you will have auto_increment_increment=2 for both masters and A would have auto_increment_offset=1 and B would have auto_increment_offset=2. For A, all auto_increment and generated sequence values will be odd.
For B, all values will be even:
See the section above.
If we change auto_increment_increment from 2 to 4 in A, it will now generate values from this sequence:
As you can see, values 3, 7, 11 are not going to be used.We can get C to use values from this sequence by configuring auto_increment_increment=4 and auto_increment_offset=3.
If we would like to add a 4'th master (D), we can do that by changing 'B' to use auto_increment_increment=4 and then configure D to have auto_increment_increment=4 and auto_increment_offset=4.
AUTO_INCREMENT columns are not permitted in , and .
If the is set, zero values will not be automatically incremented:
- an alternative to auto_increment available from
This page is licensed: GPLv2, originally from
UUID_SHORT() - Generate unique ids
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'),
('fox'),('whale'),('ostrich');SELECT * FROM animals;
+----+---------+
| id | name |
+----+---------+
| 1 | dog |
| 2 | cat |
| 3 | penguin |
| 4 | fox |
| 5 | whale |
| 6 | ostrich |
+----+---------+CREATE TABLE t (id SERIAL, c CHAR(1)) ENGINE=InnoDB;
SHOW CREATE TABLE t \G
*************************** 1. row ***************************
Table: t
Create Table: CREATE TABLE `t` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`c` char(1) DEFAULT NULL,
UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1ALTER TABLE animals AUTO_INCREMENT=8;
INSERT INTO animals (name) VALUES ('aardvark');
SELECT * FROM animals;
+----+-----------+
| id | name |
+----+-----------+
| 1 | dog |
| 2 | cat |
| 3 | penguin |
| 4 | fox |
| 5 | whale |
| 6 | ostrich |
| 8 | aardvark |
+----+-----------+
SET insert_id=12;
INSERT INTO animals (name) VALUES ('gorilla');
SELECT * FROM animals;
+----+-----------+
| id | name |
+----+-----------+
| 1 | dog |
| 2 | cat |
| 3 | penguin |
| 4 | fox |
| 5 | whale |
| 6 | ostrich |
| 8 | aardvark |
| 12 | gorilla |
+----+-----------+CREATE TABLE t (id INTEGER UNSIGNED AUTO_INCREMENT PRIMARY KEY) ENGINE = InnoDB;
INSERT INTO t VALUES (NULL);
SELECT id FROM t;
+----+
| id |
+----+
| 1 |
+----+
INSERT INTO t VALUES (10); -- higher value
SELECT id FROM t;
+----+
| id |
+----+
| 1 |
| 10 |
+----+
INSERT INTO t VALUES (2); -- lower value
INSERT INTO t VALUES (NULL); -- auto value
SELECT id FROM t;
+----+
| id |
+----+
| 1 |
| 2 |
| 10 |
| 11 |
+----+SET @@auto_increment_increment=3;
SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 3 |
| auto_increment_offset | 1 |
+--------------------------+-------+
CREATE TABLE t (c INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
INSERT INTO t VALUES (NULL), (NULL), (NULL);
SELECT * FROM t;
+---+
| c |
+---+
| 1 |
| 4 |
| 7 |
+---+
CREATE TABLE t2 (c INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
SET @@auto_increment_offset=2;
SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 3 |
| auto_increment_offset | 2 |
+--------------------------+-------+
INSERT INTO t2 VALUES (NULL), (NULL), (NULL);
SELECT * FROM t2;
+---+
| c |
+---+
| 2 |
| 5 |
| 8 |
+---+SET @@auto_increment_offset=5;
SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 3 |
| auto_increment_offset | 5 |
+--------------------------+-------+
CREATE TABLE t3 (c INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
INSERT INTO t3 VALUES (NULL), (NULL), (NULL);
SELECT * FROM t3;
+---+
| c |
+---+
| 1 |
| 4 |
| 5 |
+---+
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 3 |
| auto_increment_offset | 3 |
+--------------------------+-------+
INSERT INTO t4 VALUES (NULL), (NULL), (NULL);
SELECT * FROM t4;
+---+
| c |
+---+
| 3 |
| 6 |
| 9 |
+---+1
3
5
72
4
6
81
5
9
133
7
11CREATE OR REPLACE TABLE t1 (a INT);
INSERT t1 VALUES (0),(0),(0);
ALTER TABLE t1 MODIFY a INT NOT NULL AUTO_INCREMENT PRIMARY KEY;
SELECT * FROM t1;
+---+
| a |
+---+
| 1 |
| 2 |
| 3 |
+---+CREATE OR REPLACE TABLE t1 (a INT);
INSERT t1 VALUES (5),(0),(8),(0);
ALTER TABLE t1 MODIFY a INT NOT NULL AUTO_INCREMENT PRIMARY KEY;
SELECT * FROM t1;
+---+
| a |
+---+
| 5 |
| 6 |
| 8 |
| 9 |
+---+SET SQL_MODE='no_auto_value_on_zero';
CREATE OR REPLACE TABLE t1 (a INT);
INSERT t1 VALUES (3), (0);
ALTER TABLE t1 MODIFY a INT NOT NULL AUTO_INCREMENT PRIMARY KEY;
SELECT * FROM t1;
+---+
| a |
+---+
| 0 |
| 3 |
+---+