Reset identity seed after deleting records in SQL

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.

Answer 1

Use this stored procedure:

IF (object_id('[dbo].[pResetIdentityField]') IS NULL)
  BEGIN
    EXEC('CREATE PROCEDURE [dbo].[pResetIdentityField] AS SELECT 1 FROM DUMMY');
  END
GO

SET  ANSI_NULLS ON
GO
SET  QUOTED_IDENTIFIER ON
GO

ALTER PROCEDURE [dbo].[pResetIdentityField]
  @pSchemaName NVARCHAR(1000)
, @pTableName NVARCHAR(1000) AS
DECLARE @max   INT;
DECLARE @fullTableName   NVARCHAR(2000) = @pSchemaName + '.' + @pTableName;

DECLARE @identityColumn   NVARCHAR(1000);

SELECT @identityColumn = c.[name]
FROM sys.tables t
     INNER JOIN sys.schemas s ON t.[schema_id] = s.[schema_id]
     INNER JOIN sys.columns c ON c.[object_id] = t.[object_id]
WHERE     c.is_identity = 1
      AND t.name = @pTableName
      AND s.[name] = @pSchemaName

IF @identityColumn IS NULL
  BEGIN
    RAISERROR(
      'One of the following is true: 1. the table you specified doesn''t have an identity field, 2. you specified an invalid schema, 3. you specified an invalid table'
    , 16
    , 1);
    RETURN;
  END;

DECLARE @sqlString   NVARCHAR(MAX) = N'SELECT @maxOut = max(' + @identityColumn + ') FROM ' + @fullTableName;

EXECUTE sp_executesql @stmt = @sqlString, @params = N'@maxOut int OUTPUT', @maxOut = @max OUTPUT

IF @max IS NULL
  SET @max = 0

print(@max)

DBCC CHECKIDENT (@fullTableName, RESEED, @max)
go

--exec pResetIdentityField 'dbo', 'Table'

Just revisiting my answer. I came across a weird behaviour in sql server 2008 r2 that you should be aware of.

drop table test01

create table test01 (Id int identity(1,1), descr nvarchar(10))

execute pResetIdentityField 'dbo', 'test01'

insert into test01 (descr) values('Item 1')

select * from test01

delete from test01

execute pResetIdentityField 'dbo', 'test01'

insert into test01 (descr) values('Item 1')

select * from test01

The first select produces 0, Item 1.

The second one produces 1, Item 1. If you execute the reset right after the table is created the next value is 0. Honestly, I am not surprised Microsoft cannot get this stuff right. I discovered it because I have a script file that populates reference tables that I sometimes run after I re-create tables and sometimes when the tables are already created.

Answer 2

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

Answer 3

For a complete DELETE rows and reset the IDENTITY count, I use this (SQL Server 2008 R2)

USE mydb

-- ##################################################################################################################
-- DANGEROUS!!!! USE WITH CARE
-- ##################################################################################################################

DECLARE
  db_cursor CURSOR FOR
    SELECT TABLE_NAME
      FROM INFORMATION_SCHEMA.TABLES
     WHERE TABLE_TYPE = 'BASE TABLE'
       AND TABLE_CATALOG = 'mydb'

DECLARE @tblname VARCHAR(50)
SET @tblname = ''

OPEN db_cursor
FETCH NEXT FROM db_cursor INTO @tblname

WHILE @@FETCH_STATUS = 0
BEGIN
  IF CHARINDEX('mycommonwordforalltablesIwanttodothisto', @tblname) > 0
    BEGIN
      EXEC('DELETE FROM ' + @tblname)
      DBCC CHECKIDENT (@tblname, RESEED, 0)
    END

  FETCH NEXT FROM db_cursor INTO @tblname
END

CLOSE db_cursor
DEALLOCATE db_cursor
GO

Answer 4

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.

Answer 5

Its always better to use TRUNCATE when possible instead of deleting all records as it doesn't use log space also.

In case we need delete and need to reset the seed, always remember that if table was never populated and you used DBCC CHECKIDENT('tablenem',RESEED,0) then first record will get identity = 0 as stated on msdn documentation

In your case only rebuild the index and don't worry about losing the series of identity as this is a common scenario.

Answer 6

This is a common question and the answer is always the same: don't do it. Identity values should be treated as arbitrary and, as such, there is no "correct" order.