How to Change the Starting Number for Auto-Generated IDs in SQL Server

2. Truncate and Reseed: This approach involves two steps:

   • Truncate: This removes all rows from the table but keeps the table structure intact. Unlike deleting all rows, truncate resets the internal counters used by the identity column.
   • RESEED: After truncating, you can again use DBCC CHECKIDENT to set a new seed value for the identity column. This effectively resets the starting value for new inserts.

Important considerations:

• Reseting the identity column seed value can cause issues if the identity column is also the primary key for the table. Existing data might have references to the old identity values, leading to potential conflicts.
• Truncating the table removes all data, so make sure you have a backup if the existing data is important.

-- Example table with identity column
CREATE TABLE MyTable (
  ID int IDENTITY(1, 1) PRIMARY KEY,
  Name varchar(50)
);

-- Insert some data
INSERT INTO MyTable (Name)
VALUES ('Alice'), ('Bob'), ('Charlie');

-- Check current identity seed value
SELECT IDENT_CURRENT('MyTable'); -- This might return 3

-- Reset seed value to 100
DBCC CHECKIDENT ('MyTable', RESEED, 100);

-- Insert new row, starts with 101 (seed + increment)
INSERT INTO MyTable (Name)
VALUES ('David');

SELECT * FROM MyTable;

This code creates a table with an identity column ID. It then inserts some data and checks the current seed value. Finally, it uses DBCC CHECKIDENT with RESEED to set the seed to 100. The next inserted row will have the value 101 (100 + increment of 1).

Important: This approach doesn't change the existing identity values (1, 2, 3) in the table.

Truncate and Reseed (Resets seed value and existing identity values):

-- Same table as example 1

-- Truncate the table (removes all data and resets identity counter)
TRUNCATE TABLE MyTable;

-- Reset seed value to 5
DBCC CHECKIDENT ('MyTable', RESEED, 5);

-- Insert new data, starts with 6 (new seed + increment)
INSERT INTO MyTable (Name)
VALUES ('Alice'), ('Bob'), ('Charlie');

SELECT * FROM MyTable;

This code truncates the table, effectively resetting the internal counter for the identity column. Then, it uses DBCC CHECKIDENT to set the new seed value to 5. Now, when you insert new data, it will start with 6 (5 + increment of 1).

1. Create a New Table with New Identity:

• This approach involves creating a new table with the same structure as the original table, but with a new identity column starting from the desired seed value.
• You can then populate the new table by copying data from the original table (excluding the identity column) using techniques like INSERT INTO ... SELECT ....
• This method avoids modifying the existing data and its associated identity values. However, it requires creating and managing an additional table, which might not be ideal for all scenarios.

1. Identity Column Manipulation (Limited Use Case):

• In specific situations, you might be able to manipulate the existing identity values directly. However, this is generally not recommended and should be done with caution.
• This approach involves using advanced techniques like identity column gaps or identity gaps functionality (available in some SQL Server versions). It's crucial to understand the potential consequences and data integrity issues before attempting this method.

Important considerations for both methods:

• These approaches might require more complex logic and have potential drawbacks compared to the standard DBCC CHECKIDENT or Truncate and Reseed methods.
• Ensure you have a clear understanding of your data and table relationships before attempting any manipulation of identity values.
• Always test these methods in a non-production environment before applying them to your actual data.