Row versioning in SQL Server can be implemented using the ROWVERSION data type, which automatically generates a unique binary number that changes every time a row is updated. This is particularly useful for concurrency control, allowing you to determine whether a row has been modified since it was last read. Here’s how you can implement row versioning using the ROWVERSION data type:
Steps to Implement RowVersioning
1. Create or Modify Table to Include a ROWVERSION Column:
You can add a ROWVERSION column to a new or existing table.
Creating a new table with a ROWVERSION column:
CREATE TABLE MyTable (
ID INT PRIMARY KEY,
Name NVARCHAR(100),
UpdatedAt ROWVERSION
);
Adding a ROWVERSION column to an existing table:
ALTER TABLE MyTable
ADD UpdatedAt ROWVERSION;
2. Inserting Data:
When you insert data into the table, SQL Server automatically generates a unique value for the ROWVERSION column.
INSERT INTO MyTable (ID, Name)
VALUES (1, 'John Doe');
3. Updating Data:
When you update data, the ROWVERSION column changes automatically.
UPDATE MyTable
SET Name = 'Jane Doe'
WHERE ID = 1;
-- The UpdatedAt value will be updated automatically.
4. Using ROWVERSION for Concurrency Control:
You can use the ROWVERSION value to check whether a row has been modified since it was last read. This can be implemented in an update statement.
Example of handling concurrency:
- First, read the current value of the row including the ROWVERSION.
DECLARE @CurrentRowVersion BINARY(8);
SELECT @CurrentRowVersion = UpdatedAt
FROM MyTable
WHERE ID = 1;
- When attempting to update, include a check for the ROWVERSION value.
UPDATE MyTable
SET Name = 'John Smith'
WHERE ID = 1 AND UpdatedAt = @CurrentRowVersion;
IF @@ROWCOUNT = 0
BEGIN
-- The update failed, meaning the row has been modified by another transaction.
PRINT 'Row has been updated by another user!';
END
ELSE
BEGIN
PRINT 'Row updated successfully!';
END
Important Notes
- Data Type: ROWVERSION is synonymous with the TIMESTAMP data type in SQL Server, but it's more accurately named ROWVERSION moving forward as TIMESTAMP can be misleading (it does not represent a date and time).
- Uniqueness: ROWVERSION values are unique within a database but not across databases.
- Automatic Generation: You cannot manually insert or update a ROWVERSION column. It will always be automatically controlled by SQL Server.
- Use Case: It's particularly useful in scenarios where you need optimistic concurrency control, allowing you to handle conflicts in data efficiently when multiple users are involved.
Implementing row versioning with ROWVERSION can significantly enhance your application's handling of concurrent data access and updates by ensuring data integrity.