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Reset identity seed after deleting records in SQL Server


Question

I have inserted records into a SQL Server database table. The table had a primary key defined and the auto increment identity seed is set to “Yes”. This is done primarily because in SQL Azure, each table has to have a primary key and identity defined.

But since I have to delete some records from the table, the identity seed for those tables will be disturbed and the index column (which is auto-generated with an increment of 1) will get disturbed.

How can I reset the identity column after I deleted the records so that the column has sequence in ascending numerical order?

The identity column is not used as a foreign key anywhere in database.

2016/07/14
1
712
7/14/2016 3:48:00 PM

Accepted Answer

The DBCC CHECKIDENT management command is used to reset identity counter. The command syntax is:

DBCC CHECKIDENT (table_name [, { NORESEED | { RESEED [, new_reseed_value ]}}])
[ WITH NO_INFOMSGS ]

Example:

DBCC CHECKIDENT ('[TestTable]', RESEED, 0);
GO

It was not supported in previous versions of the Azure SQL Database but is supported now.


Thanks to Solomon Rutzky the docs for the command are now fixed.

2020/07/23
1134
7/23/2020 2:54:08 PM

DBCC CHECKIDENT ('TestTable', RESEED, 0)
GO

Where 0 is identity Start value

2015/03/26

It should be noted that IF all of the data is being removed from the table via the DELETE (i.e. no WHERE clause), then as long as a) permissions allow for it, and b) there are no FKs referencing the table (which appears to be the case here), using TRUNCATE TABLE would be preferred as it does a more efficient DELETE and resets the IDENTITY seed at the same time. The following details are taken from the MSDN page for TRUNCATE TABLE:

Compared to the DELETE statement, TRUNCATE TABLE has the following advantages:

  • Less transaction log space is used.

    The DELETE statement removes rows one at a time and records an entry in the transaction log for each deleted row. TRUNCATE TABLE removes the data by deallocating the data pages used to store the table data and records only the page deallocations in the transaction log.

  • Fewer locks are typically used.

    When the DELETE statement is executed using a row lock, each row in the table is locked for deletion. TRUNCATE TABLE always locks the table (including a schema (SCH-M) lock) and page but not each row.

  • Without exception, zero pages are left in the table.

    After a DELETE statement is executed, the table can still contain empty pages. For example, empty pages in a heap cannot be deallocated without at least an exclusive (LCK_M_X) table lock. If the delete operation does not use a table lock, the table (heap) will contain many empty pages. For indexes, the delete operation can leave empty pages behind, although these pages will be deallocated quickly by a background cleanup process.

If the table contains an identity column, the counter for that column is reset to the seed value defined for the column. If no seed was defined, the default value 1 is used. To retain the identity counter, use DELETE instead.

So the following:

DELETE FROM [MyTable];
DBCC CHECKIDENT ('[MyTable]', RESEED, 0);

Becomes just:

TRUNCATE TABLE [MyTable];

Please see the TRUNCATE TABLE documentation (linked above) for additional information on restrictions, etc.

2015/01/10

Although most answers are suggesting RESEED to 0, But many a times we need to just reseed to next Id available

declare @max int
select @max=max([Id])from [TestTable]
if @max IS NULL   //check when max is returned as null
  SET @max = 0
DBCC CHECKIDENT ('[TestTable]', RESEED,@max)

This will check the table and reset to the next ID.

2018/08/31

I tried @anil shahs answer and it reset the identity. But when a new row was inserted it got the identity = 2. So instead I changed the syntax to:

DELETE FROM [TestTable]

DBCC CHECKIDENT ('[TestTable]', RESEED, 0)
GO

Then the first row will get the identity = 1.

2018/07/25

Although most answers are suggesting RESEED to 0, and while some see this as a flaw for TRUNCATED tables, Microsoft has a solution that excludes the ID

DBCC CHECKIDENT ('[TestTable]', RESEED)

This will check the table and reset to the next ID. This has been available since MS SQL 2005 to current.

https://msdn.microsoft.com/en-us/library/ms176057.aspx

2015/08/11