A sequence is a set of integers 1, 2, 3, … that are generated in order on demand. Sequences are frequently used in databases because many applications require each row in a table to contain a unique value and sequences provide an easy way to generate them. This chapter describes how to use sequences in MySQL.
Using AUTO_INCREMENT column
The simplest way in MySQL to use Sequences is to define a column as AUTO_INCREMENT and leave rest of the things to MySQL to take care.
Example
Try out the following example. This will create table and after that it will insert few rows in this table where it is not required to give record ID because it’s auto incremented by MySQL.
mysql>CREATE TABLE Adglob_auto( id INT UNSIGNED NOT NULL AUTO_INCREMENT, name VARCHAR(30) NOT NULL,PRIMARY KEY(id)); Query OK, 0 rows affected (0.28 sec) mysql>INSERT INTO Adglob_auto(id,name) VALUES(NULL,'sai'),(NULL,'ram'); Query OK, 2 rows affected (0.12 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> SELECT * FROM insect ORDER BY id; +----+------+ | id | name | +----+------+ | 1 | sai | | 2 | ram | +----+------+ 2 rows in set (0.05 sec)
Obtain AUTO_INCREMENT Values
LAST_INSERT_ID( ) is a SQL function, so you can use it from within any client that understands how to issue SQL statements. Otherwise, PERL and PHP scripts provide exclusive functions to retrieve auto incremented value of last record.
PERL Example
Use the mysql_insertid attribute to obtain the AUTO_INCREMENT value generated by a query. This attribute is accessed through either a database handle or a statement handle, depending on how you issue the query. The following example references it through the database handle:
$dbh→do ("INSERT INTO Adglob_auto (name,date,origin)
VALUES('moth','2001-09-14','windowsill')");
my $seq = $dbh→{mysqli_insertid};
PHP Example
After issuing a query that generates an AUTO_INCREMENT value, retrieve the value by calling mysql_insert_id( ) −
mysql_query ("INSERT INTO Adglob_auto (name,date,origin)
VALUES('moth','2001-09-14','windowsill')", $conn_id);
$seq = mysqli_insert_id ($conn_id);
Renumbering an Existing Sequence
There may be a case when you have deleted many records from a table and you want to resequence all the records. This can be done by using a simple trick but you should be very careful to do so if your table is having joins with other table.
If you determine that resequencing an AUTO_INCREMENT column is unavoidable, the way to do it is to drop the column from the table, then add it again. The following example shows how to renumber the id values in the insect table using this technique −
mysql> ALTER TABLE Adglob_auto DROP id; mysql> ALTER TABLE Adglob_auto → ADD id INT UNSIGNED NOT NULL AUTO_INCREMENT FIRST, → ADD PRIMARY KEY (id);
Starting a Sequence at a Particular Value
By default, MySQLi will start sequence from 1 but you can specify any other number as well at the time of table creation. Following is the example where MySQL will start sequence from 100.
mysql> CREATE TABLE Adglob_auto → ( → id INT UNSIGNED NOT NULL AUTO_INCREMENT = 100, → PRIMARY KEY (id), → name VARCHAR(30) NOT NULL, → );
Alternatively, you can create the table and then set the initial sequence value with ALTER TABLE.
mysql> ALTER TABLE Adglob_auto AUTO_INCREMENT = 100;