How can alter existence tables to SQL Temporal table by keeping data?

Question

I have many table with data, which I want to convert to Microsoft Temporal table, But when I want to convert temporal table cause lost my data. My code is:

Alter TABLE dbo.Employee   
(    
  [EmployeeID] int NOT NULL PRIMARY KEY CLUSTERED   
  , [Name] nvarchar(100) NOT NULL  
  , [Position] varchar(100) NOT NULL   
  , [Department] varchar(100) NOT NULL  
  , [Address] nvarchar(1024) NOT NULL  
  , [AnnualSalary] decimal (10,2) NOT NULL  
  , [ValidFrom] datetime2 (2) GENERATED ALWAYS AS ROW START  
  , [ValidTo] datetime2 (2) GENERATED ALWAYS AS ROW END  
  , PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo)  
 )    
 WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.EmployeeHistory)); 

how can alter existence tables to Sql temporal table by keeping data?

Answer 1

First you should add two columns for system time period to any tables. like this:

CREATE TABLE DepartmentHistory   
(    
     DeptID int NOT NULL  
   , DeptName varchar(50) NOT NULL  
   , SysStartTime datetime2 NOT NULL  
   , SysEndTime datetime2 NOT NULL   
);   
GO   
CREATE CLUSTERED COLUMNSTORE INDEX IX_DepartmentHistory   
   ON DepartmentHistory;   
CREATE NONCLUSTERED INDEX IX_DepartmentHistory_ID_PERIOD_COLUMNS   
   ON DepartmentHistory (SysEndTime, SysStartTime, DeptID);   
GO   
CREATE TABLE Department   
(    
    DeptID int NOT NULL PRIMARY KEY CLUSTERED  
   , DeptName varchar(50) NOT NULL  
   , SysStartTime datetime2  NOT NULL  
   , SysEndTime datetime2 NOT NULL           
) ;

In the up codes, SysStartTime and SysEndTime has system period time columns.

Then you can convert them to Temporal tables easily:

ALTER TABLE dbo.Department   
   ADD PERIOD FOR SYSTEM_TIME ([SysStartTime], [SysEndTime])   
ALTER TABLE dbo.Department      
   SET (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.DepartmentHistory, DATA_CONSISTENCY_CHECK = ON));  

And, if you have temporal table and you want to edit tables schema, so you can do it by this code:

ALTER TABLE Test.dbo.Department   
   SET (SYSTEM_VERSIONING = OFF)