IDENTITY (Property) (Transact-SQL)

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IDENTITY (Property) (Transact-SQL)

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SQL Server 2005

SQL Server 2008 Books Online (October 2009)

IDENTITY (Property) (Transact-SQL)

Creates an identity column in a table. This property is used with the CREATE TABLE and ALTER TABLE Transact-SQL statements.

Note:
The IDENTITY property is different from the SQL-DMO Identity property that exposes the row identity property of a column.

Topic link icon Transact-SQL Syntax Conventions

Syntax

IDENTITY [ (seed , increment) ]

Arguments

seed: Is the value that is used for the very first row loaded into the table.

increment: Is the incremental value that is added to the identity value of the previous row that was loaded.

You must specify both the seed and increment or neither. If neither is specified, the default is (1,1).

Remarks

If an identity column exists for a table with frequent deletions, gaps can occur between identity values. If this is a concern, do not use the IDENTITY property. However, to make sure that no gaps have been created or to fill an existing gap, evaluate the existing identity values before explicitly entering one with SET IDENTITY_INSERT ON.

If you are reusing a removed identity value, use the sample code in Example B to look for the next available identity value. Replace tablename, column_type, and MAX(column_type) - 1 with a table name, identity column data type, and numeric value of the maximum allowed value (for that data type) -1.

Use DBCC CHECKIDENT to check the current identity value and compare it with the maximum value in the identity column.

If a table with an identity column is published for replication, the identity column must be managed in a way that is appropriate for the type of replication used. For more information, see Replicating Identity Columns.

Examples

A. Using the IDENTITY property with CREATE TABLE

The following example creates a new table using the IDENTITY property for an automatically incrementing identification number.

USE AdventureWorks
IF OBJECT_ID ('dbo.new_employees', 'U') IS NOT NULL
   DROP TABLE new_employees
GO
CREATE TABLE new_employees
(
 id_num int IDENTITY(1,1),
 fname varchar (20),
 minit char(1),
 lname varchar(30)
)

INSERT new_employees
   (fname, minit, lname)
VALUES
   ('Karin', 'F', 'Josephs')

INSERT new_employees
   (fname, minit, lname)
VALUES
   ('Pirkko', 'O', 'Koskitalo')

B. Using generic syntax for finding gaps in identity values

The following example shows generic syntax for finding gaps in identity values when data is removed.

Note:
The first part of the following Transact-SQL script is designed for illustration only. You can run the Transact-SQL script that starts with the comment: `-- Create the img table`.

-- Here is the generic syntax for finding identity value gaps in data.
-- The illustrative example starts here.
SET IDENTITY_INSERT tablename ON
DECLARE @minidentval column_type
DECLARE @maxidentval column_type
DECLARE @nextidentval column_type
SELECT @minidentval = MIN($IDENTITY), @maxidentval = MAX($IDENTITY)
    FROM tablename
IF @minidentval = IDENT_SEED('tablename')
   SELECT @nextidentval = MIN($IDENTITY) + IDENT_INCR('tablename')
   FROM tablename t1
   WHERE $IDENTITY BETWEEN IDENT_SEED('tablename') AND 
      @maxidentval AND
      NOT EXISTS (SELECT * FROM tablename t2
         WHERE t2.$IDENTITY = t1.$IDENTITY + 
            IDENT_INCR('tablename'))
ELSE
   SELECT @nextidentval = IDENT_SEED('tablename')
SET IDENTITY_INSERT tablename OFF
-- Here is an example to find gaps in the actual data.
-- The table is called img and has two columns: the first column 
-- called id_num, which is an increasing identification number, and the 
-- second column called company_name.
-- This is the end of the illustration example.

-- Create the img table.
-- If the img table already exists, drop it.
-- Create the img table.
IF OBJECT_ID ('dbo.img', 'U') IS NOT NULL
   DROP TABLE img
GO
CREATE TABLE img (id_num int IDENTITY(1,1), company_name sysname)
INSERT img(company_name) VALUES ('New Moon Books')
INSERT img(company_name) VALUES ('Lucerne Publishing')
-- SET IDENTITY_INSERT ON and use in img table.
SET IDENTITY_INSERT img ON

DECLARE @minidentval smallint
DECLARE @nextidentval smallint
SELECT @minidentval = MIN($IDENTITY) FROM img
 IF @minidentval = IDENT_SEED('img')
    SELECT @nextidentval = MIN($IDENTITY) + IDENT_INCR('img')
    FROM img t1
    WHERE $IDENTITY BETWEEN IDENT_SEED('img') AND 32766 AND
      NOT    EXISTS (SELECT * FROM img t2
          WHERE t2.$IDENTITY = t1.$IDENTITY + IDENT_INCR('img'))
 ELSE
    SELECT @nextidentval = IDENT_SEED('img')
SET IDENTITY_INSERT img OFF

A. Using the IDENTITY property with CREATE TABLE

B. Using generic syntax for finding gaps in identity values

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