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.
The IDENTITY property is different from the SQL-DMO Identity property that exposes the row identity property of a column.
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 a sequence generator with the NOCACHE option or use their own mechanism to generate key values.
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.
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 AdventureWorks2012 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.
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