MySQL Sequence Usage (AUTO_INCREMENT)

In MySQL, a sequence is an object that automatically generates a numeric sequence, a set of integers.1、2、3、...Because a data table can only have one auto-increment primary key field.

Although MySQL itself does not have a built-in sequence type, you can use the AUTO_INCREMENT attribute to simulate the behavior of a sequence. UsuallyAUTO_INCREMENTThe attribute is used to specify the auto-increment property of a column in a table.

An example of creating a table using AUTO_INCREMENT:

In the above example, the id column is defined as INT AUTO_INCREMENT, which means each time a row of data is inserted, the value of the id column increases automatically. The primary key constraint ensures the uniqueness of the id column.

When you insert a row of data, you can omit the id column value, and the database will automatically assign a unique, auto-incremented value to it:

INSERT INTO example_table (name) VALUES ('John');

You can also use the LAST_INSERT_ID() function to get the auto-increment value of the row just inserted:

SELECT LAST_INSERT_ID();

If you need to get the current auto-increment value of a table, you can use the following statement:

SHOW TABLE STATUS LIKE 'example_table';

In the result set, the value in the Auto_increment column is the current auto-increment value of the table.

Please note that a column using the AUTO_INCREMENT attribute must be of an integer type (usually INT or BIGINT). In addition, if you delete a row from the table, its auto-increment value will not be reused, but will continue to increase. If you wish to manually set the auto-increment value, you can use the SET statement, but this is not a common practice because it may cause uniqueness conflicts.

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Using AUTO_INCREMENT

The simplest way to use sequences in MySQL is to use MySQL AUTO_INCREMENT to define a sequence.

Example

In the following example, a data table insect is created. The id column in the insect table can grow automatically without specifying a value.

mysql> CREATE TABLE insect
    -> (
    -> id INT UNSIGNED NOT NULL AUTO_INCREMENT,
    -> PRIMARY KEY (id),
    -> name VARCHAR(30) NOT NULL, # type of insect
    -> date DATE NOT NULL, # date collected
    -> origin VARCHAR(30) NOT NULL # where collected
);
Query OK, 0 rows affected (0.02 sec)
mysql> INSERT INTO insect (id,name,date,origin) VALUES
    -> (NULL,'housefly','2001-09-10','kitchen'),
    -> (NULL,'millipede','2001-09-10','driveway'),
    -> (NULL,'grasshopper','2001-09-10','front yard');
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0
mysql> SELECT * FROM insect ORDER BY id;
+----+-------------+------------+------------+
| id | name        | date       | origin     |
+----+-------------+------------+------------+
|  1 | housefly    | 2001-09-10 | kitchen    |
|  2 | millipede   | 2001-09-10 | driveway   |
|  3 | grasshopper | 2001-09-10 | front yard |
+----+-------------+------------+------------+
3 rows in set (0.00 sec)

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Getting the AUTO_INCREMENT Value

In the MySQL client, you can use the LAST_INSERT_ID() function in SQL to get the value of the auto-increment column of the last inserted row.

Corresponding functions are also provided in PHP or PERL scripts to get the value of the auto-increment column of the last inserted row.

PERL Example

Use the mysql_insertid attribute to get the AUTO_INCREMENT value. The example is as follows:

$dbh->do ("INSERT INTO insect (name,date,origin)
VALUES('moth','2001-09-14','windowsill')");
my $seq = $dbh->{mysql_insertid};

PHP Example

PHP uses the mysql_insert_id() function to get the value of the AUTO_INCREMENT column in the executed INSERT SQL statement.

mysql_query ("INSERT INTO insect (name,date,origin)
VALUES('moth','2001-09-14','windowsill')", $conn_id);
$seq = mysql_insert_id ($conn_id);

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Resetting the Sequence

If you have deleted multiple records from a data table and want to renumber the AUTO_INCREMENT column of the remaining data, you can do so by dropping the auto-increment column and then re-adding it. However, this operation must be performed with great caution. If new records are added while deleting, data chaos may occur. The operation is as follows:

mysql> ALTER TABLE insect DROP id;
mysql> ALTER TABLE insect
    -> ADD id INT UNSIGNED NOT NULL AUTO_INCREMENT FIRST,
    -> ADD PRIMARY KEY (id);

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Setting the Starting Value of the Sequence

Normally the starting value of the sequence is 1, but if you need to specify a starting value of 100, you can achieve it with the following statement:

mysql> CREATE TABLE insect
    -> (
    -> id INT UNSIGNED NOT NULL AUTO_INCREMENT,
    -> PRIMARY KEY (id),
    -> name VARCHAR(30) NOT NULL, 
    -> date DATE NOT NULL,
    -> origin VARCHAR(30) NOT NULL
)engine=innodb auto_increment=100 charset=utf8;

Alternatively, after the table is successfully created, you can also achieve it with the following statement:

mysql> ALTER TABLE t AUTO_INCREMENT = 100;

Other Extensions