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Lesson 3: Tracking Data Changes
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FiGURe 9-15 Viewing tables with change tracking enabled
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Once you have disabled change tracking on all the tables in the database, you can disable change tracking on the database by using SSMS or the ALTER DATABASE command. The following command disables change tracking on the AdventureWorksDW2008 database:
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ALTER DATABASE AdventureWorksDW2008 SET CHANGE_TRACKING = OFF
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Understanding Additional Change Tracking Effects
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The following commands may behave differently when change tracking is enabled:
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TRUNCATE TABLE Succeeds, but deleted rows are not tracked and the minimum valid version is updated, which requires applications to reinitialize before they can resume synchronization. DROP INDEX or ALTER INDEX DISABLE
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Fails only if referencing the index that
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enforces the PRIMARY KEY constraint.
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DROP TABLE Succeeds and removes all change tracking information pertaining to the dropped table. ALTER TABLE DROP CONSTRAINT Fails if you try to drop the PRIMARY KEY constraint. You must disable change tracking before you can drop the PRIMARY KEY constraint on the table. ALTER TABLE DROP COLUMN Succeeds so long as the column is not part of the PRIMARY KEY constraint. Unless the application is programmed to handle the dropped column, data from the dropped column might still be returned as part of the change tracking information. ALTER TABLE ADD COLUMN
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Succeeds and begins tracking changes that are made to Succeeds, but data type changes on non primary key
the new column.
ALTER TABLE ALTER COLUMN
columns are not tracked.
ALTER TABLE SWITCH
Fails if one or both of the tables has change tracking enabled.
An Introduction to Microsoft SQL Server Manageability Features
Working with Change Tracking
When you are developing applications to synchronize data by using change tracking, you can use several T-SQL commands and catalog views that are provided in SQL Server 2008. The following functions allow you to query change information and manage the change tracking environment:
CHANGETABLE Used to return all change information for a table by using the CHANGES option, or change information for a specific row by using the VERSION option. CHANGE_TRACKING_MIN_VALID_VERSION Returns the minimum version that is valid for use in obtaining change tracking when you execute the CHANGETABLE function. You must specify the table object ID for the tracked table as an argument to the command.
Obtains the version that is associated with the last committed transaction. You can use this version information when you execute the CHANGETABLE function.
CHANGE_TRACKING_CURRENT_VERSION CHANGE_TRACKING_IS_COLUMN_IN_MASK Interprets the SYS_CHANGE_COLUMNS value returned by the CHANGETABLE (CHANGES . . .) function allowing your application to determine whether the column listed in the argument is included in the values returned for SYS_CHANGE_COLUMNS. WITH CHANGE_TRACKING_CONTEXT Allows you to set context information to a varbinary(128) field in the change tracking information when updates occur. This allows you to determine what application initiated an update.
cHanGe tRackinG FUnctiOnS SYntaX
More info
For detailed information about the syntax for the change tracking functions, see the article Change Tracking Functions (Transact-SQL) in SQL Server Books Online.
CHANGETABLE Function Output and Samples
The CHANGETABLE(CHANGES) function displays the following columns when queried:
SYS_CHANGE_VERSION
Displays the version of the most recent change to the row. Displays the version value of the last INSERT
SYS_CHANGE_CREATION_VERSION
operation.
SYS_CHANGE_OPERATION SYS_CHANGE_COLUMNS
Displays U for UPDATE, D for DELETE, or I for INSERT.
Displays the columns that have changed since the baseline version. This column contains NULL under the following circumstances:
The Track_Columns_Updated argument is set to OFF. The operation is an INSERT or DELETE. All non primary key columns were updated in the same operation.
Lesson 3: Tracking Data Changes
SYS_CHANGE_CONTEXT Displays a context for the update if defined. You configure the context as part of the INSERT, UPDATE, or DELETE statement by using the WITH clause. <primary key column value>
Displays the primary key value for the row.
For the following sample queries, change tracking was enabled on the AdventureWorksDW2008 database and the DimEmployee table in the AdventureWorksDW2008 database. To start with, the Track_Columns_Updated argument is set to OFF. A new row is inserted into the DimEmployee table for an employee named Jonathan Haas. His employee key is 299. Figure 9-16 displays the change by using the CHANGETABLE function. Notice that the SYS_CHANGE_VERSION is 1 and the SYS_CHANGE_OPERATION is I for INSERT.
FiGURe 9-16 Sample output from a change table query with Track_Columns_Updated off
For the next sample, two rows are updated in the DimEmployee table. Jonathan Haas, Employee 299, became a salesperson (the SalesPersonFlag column was set to 1). Syed Abbas, Employee 294, is no longer a salesperson; therefore, the SalesPersonFlag column was set to 0. Figure 9-17 shows the results of the CHANGETABLE(CHANGES) function showing all updates from the first capture to this point. Notice that the SYS_CHANGE_VERSION has been incremented to 2 for the EmployeeKey 299, but the operation is still I. The row has not been synchronized since the initial INSERT, so an INSERT still needs to occur when the table is synchronized, but the new current data with a SalesPersonFlag of 1 is included in the synchronization. For Employee 294, the SYS_CHANGE_VERSION is now set to 3 (each update to the table increments this count), the SYS_CHANGE_OPERATION is U for UPDATE, but the SYS_CHANGE_CREATION_VERSION is set to NULL because this row was inserted before change tracking was enabled. The final sample for this scenario includes the deletion of the new user, Jonathan Haas. Notice in Figure 9-18 that for the row with EmployeeKey 299, the SYS_CHANGE_VERSION has incremented to 4 and the SYS_CHANGE_OPERATION is now D for DELETE.
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