Azure, Azure SQL DB, Microsoft Technologies, T-SQL

Altering a Computed Column in a Temporal Table in Azure SQL

System-versioned temporal tables were introduced in SQL Server 2016. They provide information about data stored in the table at any point in time by storing an effective dated version of each row rather than only the data that is correct at the current time

You can alter a temporal table to add or change columns, but you must first turn off system versioning. Let’s look at an example.

CREATE TABLE [dbo].[DatabaseSize](
	 [DatabaseID] [varchar](200) NOT NULL 
	,[ServerName] [varchar](100) NOT NULL
	,[DatabaseName] [varchar](100) NOT NULL
	,[SizeBytes] [bigint] NULL
	,[SizeMB]  AS ([SizeBytes]/(1048576))
	,[ValidFrom] [datetime2](7) GENERATED ALWAYS AS ROW START NOT NULL
	,[ValidTo] [datetime2](7) GENERATED ALWAYS AS ROW END NOT NULL
	,PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo)
	,CONSTRAINT PK_DatabaseSize_DatabaseID PRIMARY KEY CLUSTERED (DatabaseID)
) WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = [dbo].[DatabaseSizeHistory]));

Temporal tables must have a primary key defined. They also must contain two datetime2 columns, declared as GENERATED ALWAYS AS ROW START / END. The statement above creates both the current table and a history table.

The history table has the same schema as the current table, with one difference: the SizeMB column in the history table is not a computed column.

The dbo.DatabaseSize table is a system-versioned table. The DatabaseSizeHistory table is the related history table. DatabaseSizeHistory contains the same columns as DatabaseSize, except the SizeMB column is not a computed column in the history table.


When I initially created the table, I typoed the formula in the computed column. You can’t alter a computed column — you must drop and recreate the column. This is no problem, just turn off system versioning and alter your table, and turn system versioning back on.

But if you try this without specifying your history table, you will find that it stops using the history table created earlier and makes a new history table.

dbo.DatabaseSize is a system versioned table. The history table now shows as dbo.MSSL_TemporalHistoryFor_1909581841

If you specify your history table while turning system versioning back on, you will encounter an error:

Setting SYSTEM_VERSIONING to ON failed because column 'SizeMB' at ordinal 5 in history table 'Test.dbo.DatabaseSizeHistory' has a different name than the column 'ValidFrom' at the same ordinal in table 'Test.dbo.databasesize'.

Temporal tables match the columns between the current table and history table not only by name and data type but by the column’s ordinal position. Dropping and adding the computed column changed its order as it was added to the end of the table.

You can change the column order of a table in the SQL Server Management Studio UI by right-clicking on the table, selecting Design, and then dragging the column to the correct position. Note that you cannot do this on the system-versioned table while system versioning is on. You can either change the column order on the history table, or turn system versioning off and then change the current table.

dragging the SizeMB column to the bottom of the columns list in the table.

Once the column orders match, you can turn system versioning back on and specify the the history table.

ALTER TABLE [dbo].[DatabaseSize]
SET (SYSTEM_VERSIONING = ON (HISTORY_TABLE = [dbo].[DatabaseSizeHistory]));

This time, the command should complete successfully. You’ll want to drop any unused auto-created history tables before you’re finished.

Azure, Azure SQL DB, Microsoft Technologies, SQL Server

New Centralized View of SQL Resources in Azure

Yesterday some new views were made available in the Azure portal that will be helpful to those of us who create or manage Azure SQL resources.

First, a new guided approach to creating resources has been added to the Azure portal. We now have a unified experience to create Azure SQL resources that offers guidance as to the type of Azure SQL resource you need for your use case: SQL database, managed instance, or SQL Server virtual machine. This new Azure SQL blade under Marketplace offers a high-level description of each offering and the scenario that it best serves. If you already know what you want but are having trouble remembering exactly what the resource is called in the marketplace, this can also alleviate that issue.

New guided experience for creating SQL Azure resources in the Azure Portal
New guided experience for creating SQL Azure resources in the Azure Portal

Notice that SQL virtual machine images are a listed offering in the new experience. As Microsoft phrased it, “SQL Server on Azure VMs is now a first-class member of the Azure SQL family.” This blade gives you an easily accessible place to see all the SQL Server VM images without having to search through lots of other unrelated VM images.

A drop-down box listing all of the available SQL Server VM images
A drop-down box listing all of the available SQL Server VM images

Once your Azure SQL resources are created, you can use the new centralized management hub to administer them. Locate the Azure SQL resources blade to see a list of all of your single databases, database servers, elastic pools, managed instances, and virtual machines running SQL.

Centralized management hub for Azure SQL resources
Centralized management hub for Azure SQL resources

This is the foundation for a unified database platform in Azure with more consistency across offerings and more manageability features to come in the future. For more information, read the announcement from Microsoft or watch the new video they posted on Channel 9.