Transact-SQL Reference

IDENTITY (Property)

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 not the same as the SQL-DMO Identity property that exposes the row identity property of a column.

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 ensure 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 check for the next available identity value. Replace tablename, column_type, and max(column_type) - 1 with your table name, identity column data type, and numeric value of the maximum allowable 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.

When the IDENTITY property is used with CREATE TABLE, Microsoft® SQL Server™ uses the NOT FOR REPLICATION option of CREATE TABLE to override the automatic incrementing of an identity column. Usually, SQL Server assigns each new row inserted in a table a value that is some increment greater than the previous highest value. However, if the new rows are replicated from another data source, the identity values must remain exactly as they were at the data source.

Examples

A. Use the IDENTITY property with CREATE TABLE

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

USE pubs
IF EXISTS(SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES
      WHERE TABLE_NAME = 'new_employees')
   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. Use generic syntax for finding gaps in identity values

This 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 purposes 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.
-- This is the beginning of the illustrative example.
SET IDENTITY_INSERT tablename ON

DECLARE @minidentval column_type
DECLARE @nextidentval column_type
SELECT @minidentval = MIN(IDENTITYCOL) FROM tablename
IF @minidentval = IDENT_SEED('tablename')
   SELECT @nextidentval = MIN(IDENTITYCOL) + IDENT_INCR('tablename')
   FROM tablename t1
   WHERE IDENTITYCOL BETWEEN IDENT_SEED('tablename') AND 
      MAX(column_type) AND
      NOT EXISTS (SELECT * FROM tablename t2
         WHERE t2.IDENTITYCOL = t1.IDENTITYCOL + 
            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 EXISTS(SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES 
      WHERE TABLE_NAME = 'img')
   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(IDENTITYCOL) FROM img
 IF @minidentval = IDENT_SEED('img')
    SELECT @nextidentval = MIN(IDENTITYCOL) + IDENT_INCR('img')
    FROM img t1
    WHERE IDENTITYCOL BETWEEN IDENT_SEED('img') AND 32766 AND
      NOT    EXISTS (SELECT * FROM img t2
          WHERE t2.IDENTITYCOL = t1.IDENTITYCOL + IDENT_INCR('img'))
 ELSE
    SELECT @nextidentval = IDENT_SEED('img')
SET IDENTITY_INSERT img OFF