LAST_INSERT_ID()SQL函数或者
mysql_insert_id()
C API 函数来获得最新插入的ID,这获取的ID是当前connection内的变量,即不会与其他连接插入的ID冲突,因此,不用担心并发插入导致的ID错误。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) ); 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 0 to the column to generate sequence numbers. If the column is declared NOT NULL
, it is also possible to assign NULL
to the column to generate sequence numbers.
You can retrieve the most recent AUTO_INCREMENT
value with the LAST_INSERT_ID()
SQL function or themysql_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:
mysql> ALTER TABLE tbl AUTO_INCREMENT = 100;
InnoDB Notes
For InnoDB
tables, be careful if you modify the column containing the auto-increment value in the middle of a sequence of INSERT
statements. For example, if you use an UPDATE
statement to put a new, larger value in the auto-increment column, a subsequent INSERT
could encounter a “Duplicate entry” error. The test whether an auto-increment value is already present occurs if you do a DELETE
followed by more INSERT
statements, or when you COMMIT
the transaction, but not after an UPDATE
statement.
MyISAM Notes
-
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
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 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.
Further Reading
More information about AUTO_INCREMENT
is available here:
-
How to assign the
AUTO_INCREMENT
attribute to a column: Section 13.1.17, “CREATE TABLE Syntax”, andSection 13.1.7, “ALTER TABLE Syntax”. -
How
AUTO_INCREMENT
behaves depending on theNO_AUTO_VALUE_ON_ZERO
SQL mode: Section 5.1.7, “Server SQL Modes”. -
How to use the
LAST_INSERT_ID()
function to find the row that contains the most recentAUTO_INCREMENT
value: Section 12.14, “Information Functions”. -
Setting the
AUTO_INCREMENT
value to be used: Section 5.1.4, “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 5.1.4, “Server System Variables”.
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