[T-SQL] DELETE, TRUNCATE and RESEED Identity

Create a temp table with Identity column beginning with value 11. The seed value is 11.

USE [TempDB]
GO-- Create TableCREATE TABLE [dbo].[TestTable]([ID] [int] IDENTITY(11,1) NOT NULL,[var] [nchar](10) NULL
) ON [PRIMARY]
GO-- Build sample dataINSERT INTO [TestTable]VALUES ('val')GO

When seed value is 11 the next value which is inserted has the identity column value as 11.

– Select Data

SELECT *FROM [TestTable]
GO


Effect of DELETE statement

-- Delete DataDELETE FROM [TestTable]
GO

When the DELETE statement is executed without WHERE clause it will delete all the rows. However, when a new record is inserted the identity value is increased from 11 to 12. It does not reset but keep on increasing.

-- Build sample dataINSERT INTO [TestTable]VALUES ('val')GO-- Select DataSELECT *FROM [TestTable]


Effect of TRUNCATE statement

-- Truncate tableTRUNCATE TABLE [TestTable]
GO

When the TRUNCATE statement is executed it will remove all the rows. However, when a new record is inserted the identity value is increased from 11 (which is original value). TRUNCATE resets the identity value to the original seed value of the table.

-- Build sample dataINSERT INTO [TestTable]VALUES ('val')GO-- Select DataSELECT *FROM [TestTable]
GO

Effect of RESEED statement

If you notice I am using the reseed value as 1. The original seed value when I created table is 11. However, I am reseeding it with value 1.

-- ReseedDBCC CHECKIDENT ('TestTable', RESEED, 1)GO

When we insert the one more value and check the value it will generate the new value as 2. This new value logic is Reseed Value + Interval Value – in this case it will be 1+1 = 2.

-- Build sample dataINSERT INTO [TestTable]VALUES ('val')GO-- Select DataSELECT *FROM [TestTable]
GO

Here is the clean up act.

-- Clean upDROP TABLE [TestTable]
GO