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IDENTITY (Property) (Transact-SQL)


THIS TOPIC APPLIES TO: yesSQL Server (starting with 2008) noAzure SQL Database noAzure SQL Data Warehouse noParallel Data Warehouse

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


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

IDENTITY [ (seed , increment) ]


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


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).

Identity columns can be used for generating key values. The identity property on a column guarantees the following:

  • Each new value is generated based on the current seed & increment.

  • Each new value for a particular transaction is different from other concurrent transactions on the table.

The identity property on a column does not guarantee the following:

  • Uniqueness of the value – Uniqueness must be enforced by using a PRIMARY KEY or UNIQUE constraint or UNIQUE index.

  • Consecutive values within a transaction – A transaction inserting multiple rows is not guaranteed to get consecutive values for the rows because other concurrent inserts might occur on the table. If values must be consecutive then the transaction should use an exclusive lock on the table or use the SERIALIZABLE isolation level.

  • Consecutive values after server restart or other failures –SQL Server might cache identity values for performance reasons and some of the assigned values can be lost during a database failure or server restart. This can result in gaps in the identity value upon insert. If gaps are not acceptable then the application should use its own mechanism to generate key values. Using a sequence generator with the NOCACHE option can limit the gaps to transactions that are never committed.

  • Reuse of values – For a given identity property with specific seed/increment, the identity values are not reused by the engine. If a particular insert statement fails or if the insert statement is rolled back then the consumed identity values are lost and will not be generated again. This can result in gaps when the subsequent identity values are generated.

These restrictions are part of the design in order to improve performance, and because they are acceptable in many common situations. If you cannot use identity values because of these restrictions, create a separate table holding a current value and manage access to the table and number assignment with your application.

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 Replicate Identity Columns.

Only one identity column can be created per table.

In memory-optimized tables the seed and increment must be set to 1,1. Setting the seed or increment to a value other than 1 results in the following error: The use of seed and increment values other than 1 is not supported with memory optimized tables.

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

USE AdventureWorks2012;

IF OBJECT_ID ('dbo.new_employees', 'U') IS NOT NULL
   DROP TABLE new_employees;
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)
   ('Karin', 'F', 'Josephs');

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

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


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.
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($IDENTITY) + IDENT_INCR('tablename')
   FROM tablename t1
      @maxidentval AND
      NOT EXISTS (SELECT * FROM tablename t2
         WHERE t2.$IDENTITY = t1.$IDENTITY + 
   SELECT @nextidentval = IDENT_SEED('tablename');
-- 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;
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.

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
      NOT    EXISTS (SELECT * FROM img t2
          WHERE t2.$IDENTITY = t1.$IDENTITY + IDENT_INCR('img'))
    SELECT @nextidentval = IDENT_SEED('img');

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