Introduction:

In SQL Server, an identity column is a column that automatically generates unique numeric values for each row inserted into a table. It is commonly used as a primary key for uniquely identifying rows in a table. However, there may be situations where you need to reset the identity column values, such as when you want to reseed the identity column to a specific value, or when you need to delete rows from a table and start the identity values from a different number. In this blog, we will explore different methods to reset identity column values in SQL Server with sample SQL table data, providing detailed explanations for each method.

Sample SQL Table Data: Let's consider a sample table called "Customers" with the following schema and data:

CREATE TABLE Customers
(
    CustomerId INT IDENTITY(1,1) PRIMARY KEY,
    FirstName NVARCHAR(50),
    LastName NVARCHAR(50),
    Email NVARCHAR(50)
);

INSERT INTO Customers (FirstName, LastName, Email)
VALUES ('John', 'Doe', '[email protected]'),
       ('Jane', 'Smith', '[email protected]'),
       ('Mark', 'Johnson', '[email protected]');

Method 1: Truncate and Re-insert Data

The first method to reset identity column values is to truncate the table and then re-insert the data. Truncating the table removes all rows from the table and resets the identity column value to its initial seed value. You can then insert the data back into the table with the desired identity column value.

-- Step 1: Truncate the table
TRUNCATE TABLE Customers;

-- Step 2: Re-insert data with desired identity column value
SET IDENTITY_INSERT Customers ON; -- Enable inserting explicit values into identity column

INSERT INTO Customers (CustomerId, FirstName, LastName, Email)
VALUES (1001, 'John', 'Doe', '[email protected]'),
       (1002, 'Jane', 'Smith', '[email protected]'),
       (1003, 'Mark', 'Johnson', '[email protected]');

SET IDENTITY_INSERT Customers OFF; -- Disable inserting explicit values into identity column

The TRUNCATE TABLE statement removes all rows from the Customers table and resets the identity column value to its initial seed value, which is 1 in this case. Then, we use the SET IDENTITY_INSERT ON statement to allow inserting explicit values into the identity column. We can then use the INSERT INTO statement to re-insert the data into the table with the desired identity column values. Finally, we use the SET IDENTITY_INSERT OFF statement to disable inserting explicit values into the identity column.

Method 2: DBCC CHECKIDENT

The second method to reset identity column values is to use the DBCC CHECKIDENT command, which allows you to manually set the current identity value of an identity column.

-- Step 1: Delete data from the table
DELETE FROM Customers;

-- Step 2: Reset identity column value
DBCC CHECKIDENT ('Customers', RESEED, 1000); -- Set the next identity value to 1000

-- Step 3: Re-insert data with desired identity column value
INSERT INTO Customers (FirstName, LastName, Email)
VALUES ('John', 'Doe', '[email protected]'),
       ('Jane', 'Smith', '[email protected]'),
       ('Mark', 'Johnson', '[email protected]');

First, we use the DELETE statement to remove all rows from the Customers table. Then, we use the DBCC CHECKIDENT command with the RESEED option to manually set the next identity value for the Customers table to 1000. This means that the next row inserted into the table will have an identity value of 1000. Finally, we use the INSERT INTO statement to re-insert the data into the table with the desired identity column values.

Method 3: Insert with explicit values

The third method to reset identity column values is to insert data into the table with explicit values for the identity column. This method allows you to specify the desired identity values directly in the INSERT INTO statement.

-- Step 1: Delete data from the table
DELETE FROM Customers;

-- Step 2: Re-insert data with desired identity column value
INSERT INTO Customers (CustomerId, FirstName, LastName, Email)
VALUES (1001, 'John', 'Doe', '[email protected]'),
       (1002, 'Jane', 'Smith', '[email protected]'),
       (1003, 'Mark', 'Johnson', '[email protected]');

First, we use the DELETE statement to remove all rows from the Customers table. Then, we use the INSERT INTO statement to re-insert the data into the table with the desired identity column values. By explicitly specifying the values for the identity column (CustomerId in this case), we can set the identity values to any desired number, effectively resetting the identity column values.

Method 4: Create a new table and copy the data

The fourth method to reset identity column values is to create a new table with the desired schema and copy the data from the original table to the new table. This method allows you to start with a clean table and set the identity column values as per your requirement.

-- Step 1: Create a new table with the desired schema
CREATE TABLE NewCustomers
(
    CustomerId INT IDENTITY(1001,1) PRIMARY KEY, -- Set the desired seed value for the identity column
    FirstName NVARCHAR(50),
    LastName NVARCHAR(50),
    Email NVARCHAR(50)
);

-- Step 2: Copy data from the original table to the new table
INSERT INTO NewCustomers (FirstName, LastName, Email)
SELECT FirstName, LastName, Email
FROM Customers;

-- Step 3: Drop the original table and rename the new table
DROP TABLE Customers;
EXEC sp_rename 'NewCustomers', 'Customers';

First, we create a new table called NewCustomers with the desired schema, including the desired seed value for the identity column (CustomerId in this case). Then, we use the INSERT INTO statement with a SELECT statement to copy the data from the original table Customers to the new table NewCustomers. Finally, we use the DROP TABLE statement to drop the original table Customers and the sp_rename system stored procedure to rename the new table NewCustomers to Customers, effectively replacing the original table with the new table.

Conclusion:

In this blog, we explored four different methods to reset identity column values in SQL Server. We discussed using the TRUNCATE TABLE statement, and DBCC CHECKIDENT command, inserting explicit values, creating a new table, and copying data. Each method has its own advantages and limitations, and you should choose the method that best fits your specific requirements and scenarios. It's important to be cautious when resetting identity column values, as it can potentially impact the integrity of your data and database operations. Always back up your data before making any changes to the identity column values or any other critical aspects of your database.