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The AUTO_INCREMENT attribute can be used to generate a unique identity for new rows:

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• Unique Key
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• Foreign Key
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• Union Auto Generate Primary Key View Free

Union Auto Generate Primary Key View Download

Learn how to define an auto increment primary key in PostgreSQL. Get instructions on learning how to use the serial data type nd how to use a custom sequence. A primary key column is identified by a primary key symbol in its row selector. If a primary key consists of more than one column, duplicate values are allowed in one column, but each combination of values from all the columns in the primary key must be unique. When you define a primary key for a table, MySQL automatically creates an index called PRIMARY. MySQL PRIMARY KEY examples. The PRIMARY KEY constraint allows you to define a primary key of a table when you create or alter table. 1) Define a PRIMARY KEY constraint in CREATE TABLE. Typically, you define the primary key for a table in the CREATE TABLE statement.

Unique Key

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No value was specified for the AUTO_INCREMENT column, so MySQL assigned sequence numbers automatically. You can also explicitly assign 0 to the column to generate sequence numbers, unless the NO_AUTO_VALUE_ON_ZERO SQL mode is enabled. For example:

If the column is declared NOT NULL, it is also possible to assign NULL to the column to generate sequence numbers. For example:

When you insert any other value into an AUTO_INCREMENT column, the column is set to that value and the sequence is reset so that the next automatically generated value follows sequentially from the largest column value. For example:

Updating an existing AUTO_INCREMENT column value also resets the AUTO_INCREMENT sequence.

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.

Use the smallest integer data type for the AUTO_INCREMENT column that is large enough to hold the maximum sequence value you will need. When the column reaches the upper limit of the data type, the next attempt to generate a sequence number fails. Use the UNSIGNED attribute if possible to allow a greater range. For example, if you use TINYINT, the maximum permissible sequence number is 127. For TINYINT UNSIGNED, the maximum is 255. See Section 11.1.2, “Integer Types (Exact Value) - INTEGER, INT, SMALLINT, TINYINT, MEDIUMINT, BIGINT” for the ranges of all the integer types.

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. This enables multiple-row inserts to be reproduced correctly on other servers in a replication setup.

To start with an AUTO_INCREMENT value other than 1, set that value with CREATE TABLE or ALTER TABLE, like this:

For information about AUTO_INCREMENT usage specific to InnoDB Ssh key generation in linux. , see Section 15.6.1.6, “AUTO_INCREMENT Handling in InnoDB”.

• 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.

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  In this case (when the AUTO_INCREMENT column is part of a multiple-column index), AUTO_INCREMENT values are reused if you delete the row with the biggest AUTO_INCREMENT value in any group. This happens even for MyISAM tables, for which AUTO_INCREMENT values normally are not reused.

• If the AUTO_INCREMENT column is part of multiple indexes, MySQL generates sequence values using the index that begins with the AUTO_INCREMENT column, if there is one. For example, if the animals table contained indexes PRIMARY KEY (grp, id) and INDEX (id), MySQL would ignore the PRIMARY KEY for generating sequence values. As a result, the table would contain a single sequence, not a sequence per grp value.

More information about AUTO_INCREMENT is available here:

• How to assign the AUTO_INCREMENT attribute to a column: Section 13.1.20, “CREATE TABLE Statement”, and Section 13.1.9, “ALTER TABLE Statement”.

• How AUTO_INCREMENT behaves depending on the NO_AUTO_VALUE_ON_ZERO SQL mode: Section 5.1.11, “Server SQL Modes”.

• How to use the LAST_INSERT_ID() function to find the row that contains the most recent AUTO_INCREMENT value: Section 12.15, “Information Functions”.

• Setting the AUTO_INCREMENT value to be used: Section 5.1.8, “Server System Variables”.

• AUTO_INCREMENT and replication: Section 17.5.1.1, “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 5.1.8, “Server System Variables”.

Auto-increment allows a unique number to be generated when a new record is inserted into a table.

AUTO INCREMENT a Field

Very often we would like the value of the primary key field to be created automatically every time a new record is inserted.

We would like to create an auto-increment field in a table.

Syntax for MySQL

The following SQL statement defines the 'P_Id' column to be an auto-increment primary key field in the 'Persons' table:

MySQL uses the AUTO_INCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTO_INCREMENT is 1, and it will increment by 1 for each new record.

To let the AUTO_INCREMENT sequence start with another value, use the following SQL statement:

To insert a new record into the 'Persons' table, we will not have to specify a value for the 'P_Id' column (a unique value will be added automatically):

The SQL statement above would insert a new record into the 'Persons' table. The 'P_Id' column would be assigned a unique value. The 'FirstName' column would be set to 'Lars' and the 'LastName' column would be set to 'Monsen'.

Syntax for SQL Server

The following SQL statement defines the 'P_Id' column to be an auto-increment primary key field in the 'Persons' table:

The MS SQL Server uses the IDENTITY keyword to perform an auto-increment feature.

By default, the starting value for IDENTITY is 1, and it will increment by 1 for each new record.

To specify that the 'P_Id' column should start at value 10 and increment by 5, change the identity to IDENTITY(10,5).

To insert a new record into the 'Persons' table, we will not have to specify a value for the 'P_Id' column (a unique value will be added automatically):

The SQL statement above would insert a new record into the 'Persons' table. The 'P_Id' column would be assigned a unique value. The 'FirstName' column would be set to 'Lars' and the 'LastName' column would be set to 'Monsen'.

Syntax for Access

The following SQL statement defines the 'P_Id' column to be an auto-increment primary key field in the 'Persons' table:

The MS Access uses the AUTOINCREMENT keyword to perform an auto-increment feature.

By default, the starting value for AUTOINCREMENT is 1, and it will increment by 1 for each new record.

To specify that the 'P_Id' column should start at value 10 and increment by 5, change the autoincrement to AUTOINCREMENT(10,5).

To insert a new record into the 'Persons' table, we will not have to specify a value for the 'P_Id' column (a unique value will be added automatically):

The SQL statement above would insert a new record into the 'Persons' table. The 'P_Id' column would be assigned a unique value. The 'FirstName' column would be set to 'Lars' and the 'LastName' column would be set to 'Monsen'.

Syntax for Oracle

In Oracle the code is a little bit more tricky.

You will have to create an auto-increment field with the sequence object (this object generates a number sequence).

Use the following CREATE SEQUENCE syntax:

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The code above creates a sequence object called seq_person, that starts with 1 and will increment by 1. It will also cache up to 10 values for performance. The cache option specifies how many sequence values will be stored in memory for faster access.

To insert a new record into the 'Persons' table, we will have to use the nextval function (this function retrieves the next value from seq_person sequence):

The SQL statement above would insert a new record into the 'Persons' table. The 'P_Id' column would be assigned the next number from the seq_person sequence. The 'FirstName' column would be set to 'Lars' and the 'LastName' column would be set to 'Monsen'.

Foreign Key

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