A temporal table is a sort of database table that is intended to track changes over time without the need for sophisticated custom programming or triggers. It accomplishes this by including system-generated columns in the table that reflect each row's valid time period. System-Versioned Temporal Tables are another name for them.

Temporal Tables with System Verification

The system automatically tracks changes to the data over time in this form of temporal table by creating a new version of each row every time it is modified or removed. The original row is kept with its valid time period, and a new version with its own valid time period is created. This enables you to view the table's history at any moment and query it as if it were a snapshot of the data at that time.

CREATE TABLE dbo.MyTemporalTable (
Id INT PRIMARY KEY,
Name VARCHAR(50),
EffectiveStartTime DATETIME2 GENERATED ALWAYS AS ROW START,
EffectiveEndTime DATETIME2 GENERATED ALWAYS AS ROW END,
PERIOD FOR SYSTEM_TIME (EffectiveStartTime, EffectiveEndTime)
) WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.MyTemporalTable_History));

How to query the temporal table?
Temporal tables, like database tables, can be queried. To query data across the main and history tables, a new clause FOR SYSTEM_TIME with five temporal-specific sub-clauses has been added. This new SELECT statement syntax is usable on a single table, via multiple joins, and via views on top of several temporal tables.

The five sub-clauses
AS OF <DateTime>
FROM <EffectiveStartTime> TO <EffectiveEndTime>
BETWEEN <EffectiveStartTime> AND <EffectiveEndTime>
CONTAINED IN (EffectiveStartTime, <EffectiveEndTime>)
ALL

-- AS OF (EffectiveStartTime<= date_time AND EffectiveEndTime> date_time)
SELECT * FROM dbo.MyTemporalTable FOR SYSTEM_TIME AS OF '2023-03-10 T10:00:00.0000000'

-- FROM (EffectiveStartTime< date_time AND EffectiveEndTime > date_time)
SELECT * FROM dbo.MyTemporalTable FOR SYSTEM_TIME FROM '2022-01-01' TO '2022-12-31'

--BETWEEN (EffectiveStartTime<= date_time AND EffectiveEndTime > date_time)
SELECT * FROM dbo.MyTemporalTable FOR SYSTEM_TIME BETWEEN '2022-01-01' AND '2022-12-31'

--CONTAINED (EffectiveStartTime>= date_time AND EffectiveEndTime <= date_time)
SELECT * FROM dbo.MyTemporalTable FOR SYSTEM_TIME CONTAINED IN ('2023-01-01', '2023-02-28')

--ALL (All rows)
SELECT * FROM dbo.MyTemporalTable FOR SYSTEM_TIME ALL

How to drop the temporal table?
Dropping a temporal table is similar to dropping any other table in your database. However, we need to consider a few things before dropping a temporal table to avoid data loss and ensure that the cleanup process is handled correctly.

Here are the general steps to drop a temporal table.

Drop the dependent objects: Before dropping the temporal table, you should first drop any dependent objects, such as triggers, indexes, or foreign keys that reference the temporal table.

Disable temporal feature: Disable the temporal feature of the table by running the ALTER TABLE statement to remove the system-versioning or application-time period specification.
ALTER TABLE dbo.MyTemporalTable SET (SYSTEM_VERSIONING = OFF);

Drop the temporal table: After removing all dependent objects and disabling the temporal feature of the table, drop the table using the DROP TABLE statement.
DROP TABLE IF EXISTS dbo.MyTemporalTable;

Limitations of temporal table

• Storage requirements: System-versioned temporal tables require additional storage to maintain historical versions of the data. The size of the history table can grow quickly, especially for tables with many updates or deletes, which can impact performance and increase storage costs.
• Query performance: Querying a temporal table can be more complex than querying a regular table, especially if you need to retrieve data from both the current and historical versions of the data. Depending on the database system, query performance may be slower for temporal tables than for regular tables.
• Configuration complexity: Setting up a temporal table requires some additional configuration steps compared to a regular table. This can make it more complex to create and maintain the table, especially if you are not familiar with the temporal table syntax.
• Compatibility: Not all database systems support temporal tables. While SQL Server, Oracle, and PostgreSQL all support temporal tables, other systems may not. This can limit your ability to use temporal tables if you need to migrate to a different database system in the future.
• Migration complexity: If you have an existing table that you want to make temporal, you may need to perform a data migration to move the existing data to the history table. This can be complex and time-consuming, especially for large tables.

Advantages of temporal table
The main advantage of a temporal table is that it allows you to maintain a full history of changes to your data over time. This provides several benefits, including:

• Auditing: With a temporal table, you can easily track changes to your data over time. This is useful for auditing purposes, as you can see when and how data was modified, deleted, or added.
• Compliance: Many industries have regulatory requirements that mandate data retention policies. By maintaining a full history of changes to your data, you can ensure compliance with these requirements.
• Recovery: In the event of data loss or corruption, a temporal table can be used to recover lost data. You can restore the table to a previous point in time and recover data that was lost or corrupted.
• Analysis: A temporal table can be used for trend analysis, as you can see how data has changed over time. This can be useful for identifying patterns, trends, and anomalies in your data.
• Data versioning: With a temporal table, you can easily maintain multiple versions of your data. This is useful if you need to maintain different versions of a record for different users or if you need to create a new version of a record without overwriting the previous version.

Overall, the temporal table provides a powerful mechanism for maintaining a full history of changes to your data. This can be useful for a variety of purposes, including auditing, compliance, recovery, analysis, and data versioning. By understanding the limitations of temporal tables, you can make an informed decision about whether they are the right solution for your needs.