SQL Server Temporal Tables: Tracking Data History Within Your Database
Managing and tracking changes in a database can be a complex task, especially when it comes to understanding how data has changed over time. Luckily, SQL Server offers a powerful feature known as temporal tables that make it much simpler to keep track of historical data. In this article, we’ll delve into what SQL Server temporal tables are, how they work, and why they might be the right solution for your data tracking needs.
What Are Temporal Tables?
Temporal tables, also referred to as system-versioned temporal tables, are a feature that was introduced in SQL Server 2016. This type of table allows you to store and manage the historical state of data over time. The system automatically maintains this history without any manual effort from users, making it an invaluable asset for auditing changes, recovering data, conducting forensics, and complying with regulations that require version control of data.
Temporal tables work by creating a pair of tables – one for the current, up-to-date data, known as the current table, and another that stores the history of changes, known as the history table. Whenever a record in the current table is modified, a copy of the original state is automatically stored in the history table before the change is applied. This means you have a full timeline of all changes made to each record in your database.
Understanding System-Versioned Temporal Tables
System-versioned temporal tables hold unique properties that distinguish them from regular tables. They have two additional datetime2 columns, the start and end times of a record’s validity, which are maintained by the system itself. These columns denote the period during which a particular record state was—or is—valid. Whenever you make changes to the data in the current table, SQL Server automatically updates these columns in both tables to reflect the change period.
SQL Server manages the relationship between the temporary table and the history table through a process called ‘system-versioning’. With system-versioning enabled, SQL Server ensures data integrity and manages the movement of data between the two tables seamlessly.
Creating Temporal Tables
Setting up temporal tables in SQL Server is relatively straightforward. First, you create a standard table, then you modify it to support system-versioning. When you set up a temporal table, you explicitly define the PERIOD FOR SYSTEM_TIME, which consists of two datetime2 columns representing the row’s valid-from and valid-to times. The history table either can be created manually or will be generated automatically by SQL Server during the system-versioning process.
CREATE TABLE Employees
(
EmployeeID int NOT NULL PRIMARY KEY CLUSTERED,
Name nvarchar(100),
Position nvarchar(100),
Department nvarchar(100),
SystemStartTime datetime2 GENERATED ALWAYS AS ROW START HIDDEN NOT NULL,
SystemEndTime datetime2 GENERATED ALWAYS AS ROW END HIDDEN NOT NULL,
PERIOD FOR SYSTEM_TIME (SystemStartTime, SystemEndTime)
) WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.EmployeeHistory));
This example creates a temporal table named ‘Employees’ with system versioning, and SQL Server automatically creates the associated history table named ‘EmployeeHistory’.
Querying Temporal Tables
Querying data from a temporal table is one of its main advantages. You can retrieve data as of a specific point in time using a straightforward extension of the familiar SELECT statement, known as the FOR SYSTEM_TIME clause. This allows you to view a snapshot of your data as it appeared at that moment.
SELECT * FROM Employees
FOR SYSTEM_TIME AS OF '2021-01-31 13:00:00'
This query will provide you with the state of the Employees table as of January 31, 2021, at 1 PM.
Managing Temporal Tables
Maintenance of temporal tables generally requires minimal effort. However, it is essential to note that the history table can grow significantly over time, which may necessitate partitioning, purging, or archiving strategies to ensure optimal database performance.
Benefits of Temporal Tables
Temporal tables offer several benefits. Auditing data change history is much easier, as is restoring historical data. Additionally, you can analyze historical trends and conduct more accurate data forensics. Lastly, compliance with regulatory standards around data versioning and history is greatly facilitated by temporal tables.
Conclusion
SQL Server’s temporal tables can be a game-changer when it comes to data management and historical tracking. They offer increased security and reliability in handling data with robust built-in support for tracking changes. Whether for auditing purposes, regulatory compliance, or simply keeping a history of your database records, temporal tables can make a vast difference in how you manage your data within SQL Server.
