When it comes to managing databases, one might encounter scenarios where resetting auto-increment identity values are required; Microsoft SQL Server is no exception. Identity values are system-generated numbers for unique identification of rows in a table, primarily used for primary key purposes. Sometimes, during the life cycle of a database, records get deleted and, in some cases, entire tables get purged, leaving gaps in identity values. When new data comes in to replace the old, it might be desirable to reset the identity values to maintain a sequential order. In SQL Server, this is performed using the DBCC CHECKIDENT command.
DBCC CHECKIDENT ('your_table', RESEED, 0);
DBCC CHECKIDENT Explained
The DBCC CHECKIDENT is a database console command (DBCC) that is used specifically for resetting identity values. The parameters for this command are fairly straightforward.
After typing DBCC CHECKIDENT, you pass in the table name – this is the table for which you want to reset the identity values. Next, use the keyword ‘RESEED’ to let SQL Server know you want to reset the values. The final parameter is the number you want to reseed the identity value to. If you want the next inserted identity value to be 1, you set the reseed value to 0. The next auto generated identity value is always one more than the reseed value.
For a clear demonstration, you consider having a table named ‘Orders’ with records having identity values 1, 2, and 3 respectively-
SELECT * FROM Orders; 1 | Order1 | 2019-11-01 2 | Order2 | 2019-11-03 3 | Order3 | 2019-11-05
Step-by-Step DBCC CHECKIDENT Application
Essentially, let’s assume we delete the 3rd record and want our next order to have an Order Id of 3. We will reseed the Orders table:
DBCC CHECKIDENT ('Orders', RESEED, 2);
That command will cause the next inserted record to have an identity value of 3.
Understanding Reseeding Gaps
An important fact to note is that DBCC RESEED does not affect existing records. For instance, in a situation where records are deleted, reseeding is practical to fill gaps. However, it can be controversial if performed when the table still has records. The reason is that if the reseed value specified is less than the current maximum identity value in the table, it would lead to duplication of identity values when inserting new records, which is counterproductive.
In summary, SQL Server’s RESEED function is a powerful tool that helps with identity value management. It must be used cautiously to maintain database integrity and to take advantage of its benefits as well.
Common Functions Related to DBCC CHECKIDENT
Here are some common functions related to DBCC CHECKIDENT:
- SCOPE_IDENTITY(): Returns the last identity value inserted into an identity column in the same scope.
- @@IDENTITY: This is a system function that returns the last-inserted identity value.
- IDENT_CURRENT(): This function returns the last identity value generated for a specific table in any session and any scope.
Using these tools effectively can significantly ease your database management efforts and improve the performance of your SQL Server databases. It’s important to familiarize ourselves with these functions as they are staples in SQL Server database management.