create code 128 barcode c# Retrieving changes to the Employees table in Visual C#.NET

Painting Code 128 Code Set A in Visual C#.NET Retrieving changes to the Employees table

Listing 9 Retrieving changes to the Employees table
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SELECT NewBaseLine = CHANGE_TRACKING_CURRENT_VERSION(), cv = SYS_CHANGE_VERSION, ccv = SYS_CHANGE_CREATION_VERSION, op = SYS_CHANGE_OPERATION, EmployeeID FROM CHANGETABLE(CHANGES dbo.Employees, <x>) AS ChT;
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The results should look something like this:
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NewBaseLine 3 3 3 cv 3 2 1 ccv NULL NULL 1 op U D I EmployeeID 2 5 6
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How does SQL Server 2008 solve these problems
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Now, the application can use this output to determine which of the following it needs to do: Reload the data from the row for EmployeeID 2, because it has been updated since the last time it was loaded. Load the new row for EmployeeID 6, because it has been created since the table was last loaded. Remove the row for EmployeeID 5 from its local cache, because it has been deleted. Record the new baseline value (3) as it will need to use this as the version to check against the next time it polls for changes. Note that change tracking does not record any information about the user who made the change. It only records the fact that a row changed. But as with SQL Server Audit, there are ways around this limitation. SQL Server 2008 supports new syntax to allow change tracking to add some contextual information to the DML statement, allowing that data to be stored along with other details of the change. This was intended to provide an application the ability to differentiate between its own updates from those of other applications, but you can use it for whatever other devious purposes you can dream up. For example, as shown in listing 10, you can easily add information such as host name and user name using an INSTEAD OF UPDATE TRIGGER, by utilizing the new WITH CHANGE_TRACKING_CONTEXT() construct, in order to store information about users performing updates to your table.
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Listing 10 Using WITH CHANGE_TRACKING_CONTEXT() in an INSTEAD OF trigger
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CREATE TRIGGER dbo.AppendEmployeeUpdates ON dbo.Employees INSTEAD OF UPDATE AS BEGIN SET NOCOUNT ON; DECLARE @i VARBINARY(128); SET @i = CONVERT ( VARBINARY(128), SUSER_SNAME() + '|' + HOST_NAME() ); WITH CHANGE_TRACKING_CONTEXT (@i) UPDATE e SET e.FirstName = i.FirstName, e.LastName = i.LastName, e.Salary = i.Salary FROM dbo.Employees e INNER JOIN inserted i ON e.EmployeeID = i.EmployeeID; END GO
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SQL Server Audit, change tracking, and change data capture
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In this case, because you are not tracking individual column updates, you don t have to worry about only updating those columns that have changed. If you do implement a solution where individual columns matter, you might want more complex logic such that the trigger only touches the base table columns that should now contain different values. And for an even more bulletproof trigger, you would also want to handle the case where the primary key might change (even though, in theory, this should never happen). You could do this in a stored procedure instead, if you can prevent direct updates to the table itself, and enforce all access via stored procedures. That is possible in some environments, but not all. Once the trigger is in place, you can run the following UPDATE statement:
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UPDATE dbo.Employees SET LastName = 'Malone' WHERE EmployeeID = 2;
And now when you call the CHANGETABLE function, as shown in listing 11, you can add a new column that will return that contextual information (assuming the existing baseline was 3 after the above statements).
Listing 11 Calling the CHANGETABLE function
SELECT NewBaseLine = CHANGE_TRACKING_CURRENT_VERSION(), [user|host] = CONVERT(NVARCHAR(128), SYS_CHANGE_CONTEXT), cv = SYS_CHANGE_VERSION, ccv = SYS_CHANGE_CREATION_VERSION, op = SYS_CHANGE_OPERATION, EmployeeID FROM CHANGETABLE(CHANGES dbo.Employees, 3) AS ChT; -- results: NewBaseLine user|host cv 4 SENTINEL\Aaron|SENTINEL 11 ccv NULL op EmployeeID U 2
Arguably, you could also use the trigger to store the old and new values off in a table somewhere for deferred analysis. But that would require you to manually create tables to capture all of that information, and come up with your own cleanup mechanism. And, without spoiling any surprises, you would be duplicating the functionality of another feature added in SQL Server 2008. Before proceeding, you can disable change tracking on the HR database and the dbo.Employees table using the code in listing 12.
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