How to Reset Identity Column Values in SQL Server

Introduction

In SQL Server, an identity column is a column that automatically generates a unique value for each new row inserted into a table. It is commonly used as a primary key or to maintain a sequential record of inserted data. Occasionally, you may need to reset the identity column values, for example, after truncating the table or reseeding the identity column. This documentation explains the process of resetting identity column values in SQL Server.

Prerequisites

Before resetting the identity column, ensure that you have the necessary permissions to alter the table and modify its properties.

Methods to Reset Identity Column

There are two main methods to reset identity column values in SQL Server:

Method 1: TRUNCATE TABLE and DBCC CHECKIDENT

This method involves using the TRUNCATE TABLE statement to remove all data from the table and then using the DBCC CHECKIDENT command to reseed the identity column.

Step 1: TRUNCATE TABLE

First, use the TRUNCATE TABLE statement to remove all data from the table while preserving the table structure and the identity column:

sql
TRUNCATE TABLE YourTableName;

Step 2: DBCC CHECKIDENT

Next, use the DBCC CHECKIDENT command to reset the identity column value to the desired seed:

sql
DBCC CHECKIDENT ('YourTableName', RESEED, NewSeedValue);

Replace YourTableName with the actual name of your table, and NewSeedValue with the value you want to set as the new seed for the identity column.

Method 2: SET IDENTITY_INSERT

This method involves using the SET IDENTITY_INSERT statement to allow explicit values to be inserted into the identity column temporarily.

Step 1: SET IDENTITY_INSERT ON

Before inserting data, use the SET IDENTITY_INSERT statement to allow explicit values in the identity column:

sql
SET IDENTITY_INSERT YourTableName ON;

Step 2: Insert Data

Insert the data into the table, making sure to provide explicit values for the identity column:

sql
INSERT INTO YourTableName (IdentityColumn, OtherColumns)
VALUES (ExplicitValue, OtherValues);

Step 3: SET IDENTITY_INSERT OFF

After inserting the data, remember to turn off the SET IDENTITY_INSERT:

sql
SET IDENTITY_INSERT YourTableName OFF;

Replace YourTableName with the actual name of your table and adjust the column names and values accordingly.

Example

Let's illustrate Method 1 with an example:

Suppose we have a table called Employees with an identity column named EmployeeID. We want to reset the identity column to start from 1.

sql
-- Step 1: Truncate the table
TRUNCATE TABLE Employees;

-- Step 2: Reset the identity column
DBCC CHECKIDENT ('Employees', RESEED, 1);

After executing these commands, the EmployeeID column will start generating values from 1 for each new row inserted into the Employees table.

Conclusion

Resetting identity column values in SQL Server can be achieved using either the TRUNCATE TABLE and DBCC CHECKIDENT method or the SET IDENTITY_INSERT method. Choose the appropriate method based on your specific requirements and preferences. Always take a backup of your data before performing any changes to avoid data loss.

Remember to exercise caution while altering the identity column values, as resetting the seed can lead to data inconsistencies if not done properly.