Skip to main content
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 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:

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 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 mysql_insert_id( ):
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 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 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 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 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;

Comments

Post a Comment

Popular posts from this blog

In this chapter, we will see how to enable remote desktop application. It is important because this enables us to work remotely on the server. To do this, we have the following two options. For the first option, we have to follow the steps given below. Step 1  − Go to Start → right click “This PC” → Properties. Step 2  − On Left side click “Remote Setting”. Step 3  − Check radio button “Allow Remote connection to this computer” and Check box “Allow connection only from computers running Remote Desktop with Network Level Authentication (recommended)” → click “Select Users”. Step 4  − Click Add. Step 5  − Type user that you want to allow access. In my case, it is administrator → click OK. For the  second option , we need to follow the steps given below. Step 1  − Click on “Server Manage” → Local Server → click on “Enable” or Disable, if it is Disabled.
Post a Comment
Read more
The table creation command requires: Name of the table Names of fields Definitions for each field Syntax: Here is generic SQL syntax to create a MySQL table: CREATE TABLE table_name ( column_name column_type ); Now, we will create following table in  TUTORIALS  database. tutorials_tbl ( tutorial_id INT NOT NULL AUTO_INCREMENT , tutorial_title VARCHAR ( 100 ) NOT NULL , tutorial_author VARCHAR ( 40 ) NOT NULL , submission_date DATE , PRIMARY KEY ( tutorial_id ) ); Here few items need explanation: Field Attribute  NOT NULL  is being used because we do not want this field to be NULL. So if user will try to create a record with NULL value, then MySQL will raise an error. Field Attribute  AUTO_INCREMENT  tells MySQL to go ahead and add the next available number to the id field. Keyword  PRIMARY KEY  is used to define a column as primary key. You can use multiple columns separated by comma to define...
Post a Comment
Read more
In this chapter, we will see how to configure WSUS and tune it. The following steps should be followed for configuring it. Step 1  − When you open it for the first time, you should do it by going to “Server Manager” → Tools → Windows Server Update Services, then a Configuration wizard will be opened and then click → Next. Step 2  − Click “Start Connecting” → Wait until the green bar is full and then → Next. Step 3  − Check the box for which the updates want to be taken, I did for English and then → Next. Step 4  − Check the box for all the products which you want to update. It is just for Microsoft products and it is recommended to include all the products related to Microsoft and then → Next. Step 5  − Choose the classification updated to be downloaded, if you have a very good internet speed, then check all the boxes, otherwise just check “Critical Updates”. Step 6  − Now we should schedule the updates which I will recommend to do it a...
Post a Comment
Read more