c# datamatrix open source WORKING WITH THE DATA in Visual C#.NET

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CHAPTER 8 WORKING WITH THE DATA
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UPDATE ShareDetails.Shares SET CurrentPrice = CurrentPrice * 1.1 -- WHERE ShareId = 3 SELECT 'Within the transaction',ShareId,ShareDesc,CurrentPrice FROM ShareDetails.Shares ROLLBACK TRAN SELECT 'After',ShareId,ShareDesc,CurrentPrice FROM ShareDetails.Shares -- WHERE ShareId = 3 4. The results, as you see in Figure 8-48, show us exactly what has happened. Take a moment to look over these results. The first list shows the full set of rows in the ShareDetails.Shares table prior to our UPDATE. The middle recordset shows us the BEGIN transaction where we have updated every share, and the final listing shows the data restored back to its original state via a ROLLBACK TRAN.
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Figure 8-48. Updating with transaction label and a ROLLBACK TRAN
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Nested Transactions
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Let s look at one last example before moving on. It is possible to nest transactions inside one another. We touch on this enough for you to have a good understanding on nested transactions, but this is not a complete coverage, as it can get very complex and messy if you involve save points, stored procedures, triggers, and so on. The aim of this section is to give you an understanding of the basic but crucial points of how nesting transactions work. Nested transactions can occur in a number of different scenarios. For example, you could have a transaction in one set of code in a stored procedure, which calls a second stored procedure that also has a transaction. We will look at a simpler scenario where we just keep the transactions in one set of code.
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CHAPTER 8 WORKING WITH THE DATA
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What you need to be clear about is how the ROLLBACK and COMMIT TRAN commands work in a nested transaction. First of all, let s see what we mean by nesting a simple transaction. The syntax is shown here, and you can see that two BEGIN TRAN statements occur before you get to a COMMIT or a ROLLBACK: BEGIN TRAN Statements BEGIN TRAN Statements COMMIT|ROLLBACK TRAN COMMIT|ROLLBACK TRAN As each transaction commences, SQL Server adds 1 to a running count of transactions it holds in a system variable called @@TRANCOUNT. Therefore, as each BEGIN TRAN is executed, @@TRANCOUNT increases by 1. As each COMMIT TRAN is executed, @@TRANCOUNT decreases by 1. It is not until @@TRANCOUNT is at a value of 1 that you can actually commit the data to the database. The code that follows might help you to understand this a bit more. Enter and execute this code and take a look at the output, which should resemble Figure 8-49. The first BEGIN TRAN increases @@TRANCOUNT by 1, as does the second BEGIN TRAN. The first COMMIT TRAN marks the changes to be committed, but does not actually perform the changes because @@TRANCOUNT is 2. It simply creates the correct BEGIN/COMMIT TRAN nesting and reduces @@TRANCOUNT by 1. The second COMMIT TRAN will succeed and will commit the data, as @@TRANCOUNT is 1. BEGIN TRAN ShareUpd SELECT '1st TranCount',@@TRANCOUNT BEGIN TRAN ShareUpd2 SELECT '2nd TranCount',@@TRANCOUNT COMMIT TRAN ShareUpd2 SELECT '3rd TranCount',@@TRANCOUNT COMMIT TRAN -- It is at this point that data modifications will be committed SELECT 'Last TranCount',@@TRANCOUNT
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Figure 8-49. Showing the @@TRANCOUNT
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Note After the last COMMIT TRAN, @@TRANCOUNT is at 0. Any further instances of COMMIT TRAN or
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ROLLBACK TRAN will generate an error.
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CHAPTER 8 WORKING WITH THE DATA
If in the code there is a ROLLBACK TRAN, then the data is immediately rolled back no matter where you are within the nesting, and @@TRANCOUNT is set to 0. Therefore, any further ROLLBACK TRAN or COMMIT TRAN instances will fail, so you do need to have error handling, which we look at in 11. Try to avoid nesting transactions where possible, especially when one stored procedure calls another stored procedure within a transaction. It is not wrong, but it does require a great deal of care. Now that updating data has been completed, the only area of data manipulation left is row deletion, which we look at now.
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