mysql的AUTO_INCREMENT

来自手册

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

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.Using AUTO_INCREMENT

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This enables multiple-row inserts to be reproduced correctly on other servers in

a replication setup.

注：

1.可以使用 sql函数 LAST_INSERT_ID()  或者 C API函数  mysql_insert_id()来获取最近插入的值的id,这种方法是线程安全的，所以不会被其他的插入影响到。

2.一次插入多条记录时，获取到的是 第一条插入的自增id

3.修改自增id的方法：ALTER TABLE tbl_name AUTO_INCREMENT=100; 注：AUTO_INCREMENT需要大写，当修改的值小于现有id时，修改无效。

4.可以使用UNSIGNED来增加可用数量

5.可以使用 SHOW CREATE TABLE tbl_name；来查看自增id的当前值

6.当AUTO_INCREMENT超过范围时，insert会提示错误：

Failed to read auto-increment value from storage engine

更多待续：

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.

For more information about AUTO_INCREMENT and InnoDB, see Section 14.5.5, “AUTO_INCREMENT

Handling in InnoDB”.

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.

在MyISAM引擎中，可以使用自增字段和另外一个字段联合做主键，这样自增字段会根据另一个字段的值，来做分组的自增，如：

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 |
+--------+----+---------+

Further Reading

More information about AUTO_INCREMENT is available here:

• How to assign the AUTO_INCREMENT attribute to a column: Section 13.1.14, “CREATE TABLE

Syntax”, and Section 13.1.6, “ALTER TABLE Syntax”.

• How AUTO_INCREMENT behaves depending on the NO_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 recent

AUTO_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 and

auto_increment_offset) that can be used for replication: Section 5.1.4, “Server System

Variables”