Managing Identity Columns in SQL Server

Understanding Identity Columns

In SQL Server, identity columns are automatically incrementing numerical values typically used as primary keys. These columns simplify the process of generating unique identifiers for each row in a table. However, situations arise where you might need to manage or reset the identity seed, particularly after deleting records. This tutorial explores how to effectively manage identity columns to maintain data integrity and predictable sequence generation.

Why Reset the Identity Seed?

After deleting records from a table with an identity column, gaps may appear in the sequence of identifiers. While not inherently problematic, these gaps can sometimes be undesirable, especially if the identity column is used in applications that expect a continuous sequence. Resetting the identity seed allows you to start the sequence again from a specific value, ensuring a consistent numbering scheme for newly inserted records.

Using DBCC CHECKIDENT to Reseed

SQL Server provides the DBCC CHECKIDENT command specifically for managing identity columns. This command allows you to view the current identity value and, crucially, to reseed the identity column to a new starting value.

The basic syntax is:

DBCC CHECKIDENT (table_name, { NORESEED | RESEED [, new_reseed_value ]}) [WITH NO_INFOMSGS]

• table_name: The name of the table containing the identity column.
• NORESEED: Prevents reseeding of the identity column.
• RESEED: Reseeds the identity column.
• new_reseed_value: The new starting value for the identity column. If omitted, the seed is set to 1.
• WITH NO_INFOMSGS: Suppresses informational messages.

Example:

To reseed the identity column of a table named TestTable to 0, you would use the following statement:

DBCC CHECKIDENT ('TestTable', RESEED, 0);
GO

This will set the next identity value to 1. It’s common to reseed to 0 to ensure the next inserted row receives an identity of 1.

Important Considerations:

• Reseeding to a value lower than the current maximum identity value can lead to primary key conflicts if you insert new records before addressing the existing ones. Use caution when choosing the new_reseed_value.
• Ensure you understand the implications of reseeding, especially if the identity column is used as a foreign key in other tables.

Dynamically Determining the Reseed Value

In scenarios where you want to reseed to the next available identity value, you can dynamically determine this value using a query:

declare @max int
select @max=max([Id]) from [TestTable]
if @max IS NULL   --check when max is returned as null
  SET @max = 0
DBCC CHECKIDENT ('[TestTable]', RESEED, @max);

This code snippet first retrieves the maximum identity value from the table. If the table is empty (resulting in a NULL value), it sets @max to 0. Then, it reseeds the identity column to the value of @max, ensuring the next inserted record receives the next sequential ID.

Optimizing with TRUNCATE TABLE

If you intend to completely remove all data from a table, consider using the TRUNCATE TABLE statement instead of DELETE. TRUNCATE TABLE is significantly faster and more efficient because it deallocates data pages rather than deleting rows individually.

Critically, TRUNCATE TABLE automatically resets the identity seed to the initial seed value (or 1 if no seed is defined).

Example:

TRUNCATE TABLE [MyTable];

Important Considerations with TRUNCATE TABLE:

• TRUNCATE TABLE cannot be used if foreign key constraints reference the table.
• The operation cannot be rolled back.

Best Practices

• Understand the Implications: Carefully consider the impact of reseeding on your application and data integrity.
• Backup Your Data: Before performing any operation that modifies your data, it’s crucial to create a backup to prevent data loss.
• Consider Alternatives: Evaluate whether reseeding is truly necessary. Sometimes, gaps in identity values are acceptable and do not require intervention.
• Use TRUNCATE TABLE when appropriate: If you want to remove all rows from a table and reset the identity seed, TRUNCATE TABLE is the most efficient option, as long as foreign key constraints are not present.