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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:
- );
- ('dog'),('cat'),('penguin'),
- ('lax'),('whale'),('ostrich');
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:
If the column is declared NOT NULL
, it is
also possible to assign NULL
to the column to
generate sequence numbers. For example:
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:
- +-----+-----------+
- | 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 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.
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:
InnoDB Notes
For information about AUTO_INCREMENT
usage
specific to InnoDB
, see
Section 15.6.1.4, “AUTO_INCREMENT Handling in InnoDB”.
For
MyISAM
tables, you can specifyAUTO_INCREMENT
on a secondary column in a multiple-column index. In this case, the generated value for theAUTO_INCREMENT
column is calculated asMAX(
. This is useful when you want to put data into ordered groups.auto_increment_column
) + 1 WHERE prefix=given-prefix
- ('mammal','dog'),('mammal','cat'),
- ('bird','penguin'),('fish','lax'),('mammal','whale'),
- ('bird','ostrich');
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 biggestAUTO_INCREMENT
value in any group. This happens even forMyISAM
tables, for whichAUTO_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 theAUTO_INCREMENT
column, if there is one. For example, if theanimals
table contained indexesPRIMARY KEY (grp, id)
andINDEX (id)
, MySQL would ignore thePRIMARY KEY
for generating sequence values. As a result, the table would contain a single sequence, not a sequence pergrp
value.
More information about AUTO_INCREMENT
is
available here:
How to assign the
AUTO_INCREMENT
attribute to a column: Section 13.1.20, “CREATE TABLE Syntax”, and Section 13.1.9, “ALTER TABLE Syntax”.How
AUTO_INCREMENT
behaves depending on theNO_AUTO_VALUE_ON_ZERO
SQL mode: Section 5.1.11, “Server SQL Modes”.How to use the
LAST_INSERT_ID()
function to find the row that contains the most recentAUTO_INCREMENT
value: Section 12.15, “Information Functions”.Setting the
AUTO_INCREMENT
value to be used: Section 5.1.8, “Server System Variables”.AUTO_INCREMENT
and replication: Section 17.4.1.1, “Replication and AUTO_INCREMENT”.Server-system variables related to
AUTO_INCREMENT
(auto_increment_increment
andauto_increment_offset
) that can be used for replication: Section 5.1.8, “Server System Variables”.
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Document créé le 26/06/2006, dernière modification le 26/10/2018
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