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3.6.9 Using AUTO_INCREMENT

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

  1. CREATE TABLE animals (
  2.      id MEDIUMINT NOT NULL AUTO_INCREMENT,
  3.      name CHAR(30) NOT NULL,
  4.      PRIMARY KEY (id)
  5. );
  6.  
  7. INSERT INTO animals (name) VALUES
  8.     ('dog'),('cat'),('penguin'),
  9.     ('lax'),('whale'),('ostrich');
  10.  
  11. 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 0 to the column to generate sequence numbers, unless the NO_AUTO_VALUE_ON_ZERO SQL mode is enabled. For example:

  1. 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:

  1. 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:

  1. INSERT INTO animals (id,name) VALUES(100,'rabbit');
  2. INSERT INTO animals (id,name) VALUES(NULL,'mouse');
  3. SELECT * FROM animals;
  4. +-----+-----------+
  5. | id  | name      |
  6. +-----+-----------+
  7. |   1 | dog       |
  8. |   2 | cat       |
  9. |   3 | penguin   |
  10. |   4 | lax       |
  11. |   5 | whale     |
  12. |   6 | ostrich   |
  13. |   7 | groundhog |
  14. |   8 | squirrel  |
  15. | 100 | rabbit    |
  16. | 101 | mouse     |
  17. +-----+-----------+

Updating an existing AUTO_INCREMENT column value also resets the AUTO_INCREMENT sequence.

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 11.2.1, “Integer Types (Exact Value) - INTEGER, INT, SMALLINT, TINYINT, MEDIUMINT, BIGINT” for the ranges of all the integer types.

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.

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

  1. mysql> ALTER TABLE tbl AUTO_INCREMENT = 100;

InnoDB Notes

For information about AUTO_INCREMENT usage specific to InnoDB, see Section 15.6.1.4, “AUTO_INCREMENT Handling in InnoDB”.

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MyISAM Notes

  • For MyISAM 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.

    1. CREATE TABLE animals (
    2.     grp ENUM('fish','mammal','bird') NOT NULL,
    3.     id MEDIUMINT NOT NULL AUTO_INCREMENT,
    4.     name CHAR(30) NOT NULL,
    5.     PRIMARY KEY (grp,id)
    6. ) ENGINE=MyISAM;
    7.  
    8. INSERT INTO animals (grp,name) VALUES
    9.     ('mammal','dog'),('mammal','cat'),
    10.     ('bird','penguin'),('fish','lax'),('mammal','whale'),
    11.     ('bird','ostrich');
    12.  
    13. 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 generates 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.

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Further Reading

More information about AUTO_INCREMENT is available here:


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Document created the 26/06/2006, last modified the 26/10/2018
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