What Is SQL Sequence? | SQL Tutorial

SQL Sequence := Sequence is a feature supported by some database systems to produce unique values on demand. Some DBMS like MySQL supports AUTO_INCREMENT in place of Sequence. AUTO_INCREMENT is applied on columns, it automatically increments the column value by 1 each time a new record is entered into the table. Sequence is also some what similar to AUTO_INCREMENT but its has some extra features.

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 the rest to MySQL to take care.

Example

Try out the following example. This will create a table and after that it will insert a few rows in this table where it is not required to give a record ID because its auto-incremented by MySQL.

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)

Obtain AUTO_INCREMENT Values

The LAST_INSERT_ID( ) is an 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 INSECT (name,date,origin)
VALUES('moth','2001-09-14','windowsill')");
my $seq = $dbh->{mysql_insertid};

PHP Example

After issuing a query that generates an AUTO_INCREMENT value, retrieve the value by calling the mysql_insert_id( ) function.

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

Renumbering an Existing Sequence

There may be a case when you have deleted many records from a table and you want to re-sequence all the records. This can be done by using a simple trick, but you should be very careful to do this and check if your table is having a join with another table or not.

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

Starting a Sequence at a Particular Value

By default, MySQL will start the sequence from 1, but you can specify any other number as well at the time of table creation.

The following code block has an example where MySQL will start sequence from 100.

mysql> CREATE TABLE INSECT
   -> (
   -> id INT UNSIGNED NOT NULL AUTO_INCREMENT = 100,
   -> PRIMARY KEY (id),
   -> name VARCHAR(30) NOT NULL, # type of insect
   -> date DATE NOT NULL, # date collected
   -> origin VARCHAR(30) NOT NULL # where collected
);

Alternatively, you can create the table and then set the initial sequence value with ALTER TABLE.

mysql> ALTER TABLE t AUTO_INCREMENT = 100;

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Creating Sequence

Syntax to create sequences is,

CREATE Sequence sequence-name
start with initial-value
increment by increment-value
maxvalue maximum-value
cycle|nocycle 

initial-value specifies the starting value of the Sequence, increment-value is the value by which sequence will be incremented and maxvalue specifies the maximum value until which sequence will increment itself.cycle specifies that if the maximum value exceeds the set limit, sequence will restart its cycle from the begining. No cycle specifies that if sequence exceeds maxvalue an error will be thrown.

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Example to create Sequence

The sequence query is following

CREATE Sequence seq_1
start with 1
increment by 1
maxvalue 999
cycle ;

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Example to use Sequence

The class table,

ID	NAME
1	abhi
2	adam
4	alex

The sql query will be,

INSERT into class value(seq_1.nextval,'anu');

Result table will look like,

ID	NAME
1	abhi
2	adam
4	alex
1	anu

Once you use nextval the sequence will increment even if you don’t Insert any record into the table.

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