Querying data in a system-versioned temporal table
When you want to get latest (current) state of data in a temporal table, you can query the same way as you query a non-temporal table. If the
PERIOD columns aren't hidden, their values will appear in a
SELECT * query. If you specified
PERIOD columns as
HIDDEN, their values won't appear in a
SELECT * query. When the
PERIOD columns are hidden, you must reference the
PERIOD columns specifically in the
SELECT clause to return the values for these columns.
To perform any type of time-based analysis, use the new
FOR SYSTEM_TIME clause with four temporal-specific subclauses to query data across the current and history tables. For more information on these clauses, see Temporal Tables and FROM (Transact-SQL)
AS OF <date_time>
FROM <start_date_time> TO <end_date_time>
BETWEEN <start_date_time> AND <end_date_time>
CONTAINED IN (<start_date_time>, <end_date_time>)
FOR SYSTEM_TIME can be specified independently for each table in a query. It can be used inside common table expressions, table-valued functions, and stored procedures. When using a table alias with a temporal table, the
FOR SYSTEM_TIME clause must be included between the temporal table name and the alias (see "Query for a specific time using the AS OF subclause" second example, below).
Query for a specific time using the
AS OF subclause
AS OF subclause when you need to reconstruct state of data as it was at any specific time in the past. You can reconstruct the data with the precision of datetime2 type that was specified in
PERIOD column definitions.
AS OF subclause clause can be used with constant literals or with variables, which allows you to dynamically specify time condition. The values provided are interpreted as UTC time.
This first example returns the state of the dbo.Department table
AS OF a specific date in the past.
/*State of entire table AS OF specific date in the past*/ SELECT [DeptID], [DeptName], [ValidFrom], [ValidTo] FROM [dbo].[Department] FOR SYSTEM_TIME AS OF '2021-09-01 T10:00:00.7230011';
This second example compares the values between two points in time for a subset of rows.
DECLARE @ADayAgo datetime2 SET @ADayAgo = DATEADD(day, -1, sysutcdatetime()) /*Comparison between two points in time for subset of rows*/ SELECT D_1_Ago.[DeptID], D.[DeptID], D_1_Ago.[DeptName], D.[DeptName], D_1_Ago.[ValidFrom], D.[ValidFrom], D_1_Ago.[ValidTo], D.[ValidTo] FROM [dbo].[Department] FOR SYSTEM_TIME AS OF @ADayAgo AS D_1_Ago JOIN [Department] AS D ON D_1_Ago.[DeptID] = [D].[DeptID] AND D_1_Ago.[DeptID] BETWEEN 1 and 5;
Using views with
AS OF subclause in temporal queries
Using views is useful in scenarios when complex point-in time analysis is required. A common example is generating a business report today with the values for previous month.
Usually, customers have a normalized database model, which involves many tables with foreign key relationships. Finding out how data from that normalized model looked at a point in the past can be challenging, since all tables change independently on their own cadence.
In this case, the best option is to create a view and apply the
AS OF subclause to the entire view. Using this approach allows you to decouple modeling of the data access layer from point-in time analysis, as SQL Server will apply
AS OF clause transparently to all temporal tables that participate in view definition. Furthermore, you can combine temporal with non-temporal tables in the same view and
AS OF will be applied only to temporal ones. If the view doesn't reference at least one temporal table, applying temporal querying clauses to it will fail with an error.
/* Create view that joins three temporal tables: Department, CompanyLocation, LocationDepartments */ CREATE VIEW [dbo].[vw_GetOrgChart] AS SELECT [CompanyLocation].LocID , [CompanyLocation].LocName , [CompanyLocation].City , [Department].DeptID , [Department].DeptName FROM [dbo].[CompanyLocation] LEFT JOIN [dbo].[LocationDepartments] ON [CompanyLocation].LocID = LocationDepartments.LocID LEFT JOIN [dbo].[Department] ON LocationDepartments.DeptID = [Department].DeptID; GO /* Querying view AS OF */ SELECT * FROM [vw_GetOrgChart] FOR SYSTEM_TIME AS OF '2021-09-01 T10:00:00.7230011';
Query for changes to specific rows over time
The temporal subclauses
FROM ... TO,
BETWEEN ... AND and
CONTAINED IN are useful when you need to get all historical changes for a specific row in the current table (also known as a data audit).
The first two subclauses return row versions that overlap with a specified period (that is, those that started before given period and ended after it), while
CONTAINED IN returns only those that existed within specified period boundaries.
If you search for non-current row versions only, you should query the history table directly, as this will yield the best query performance. Use
ALL when you need to query current and historical data without any restrictions.
/* Query using BETWEEN...AND sub-clause*/ SELECT [DeptID] , [DeptName] , [ValidFrom] , [ValidTo] , IIF (YEAR(ValidTo) = 9999, 1, 0) AS IsActual FROM [dbo].[Department] FOR SYSTEM_TIME BETWEEN '2021-01-01' AND '2021-12-31' WHERE DeptId = 1 ORDER BY ValidFrom DESC; /* Query using CONTAINED IN sub-clause */ SELECT [DeptID], [DeptName], [ValidFrom],[ValidTo] FROM [dbo].[Department] FOR SYSTEM_TIME CONTAINED IN ('2021-04-01', '2021-09-25') WHERE DeptId = 1 ORDER BY ValidFrom DESC; /* Query using ALL sub-clause */ SELECT [DeptID] , [DeptName] , [ValidFrom] , [ValidTo] , IIF (YEAR(ValidTo) = 9999, 1, 0) AS IsActual FROM [dbo].[Department] FOR SYSTEM_TIME ALL ORDER BY [DeptID], [ValidFrom] DESC;