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Locking Data
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The whole area of locking data, how locks are held, and how to avoid problems with them, is a very large complex area and not for the fainthearted. However, it is necessary to be aware of locks, and at least have a small amount of background knowledge on them so that when you design your queries you stand a chance of avoiding problems. The basis of locking is to allow one transaction to update data, knowing that if it has to roll back any changes, no other transaction has modified the data since the first transaction did. To explain this with an example, if you have a transaction that updates the CustomerDetails. Customers table, and then moves on to update the TransactionDetails.Transactions table, but hits a problem when updating the TransactionDetails.Transactions table, the transaction must be safe in the knowledge that it is only rolling back the changes it made, and not changes by another transaction. Therefore, until all the table updates within the transaction are either successfully completed or have been rolled back, the transaction will keep hold of any data inserted, modified, or deleted. However, one problem with this approach is that SQL Server may not just hold the data that the transaction has modified. Keeping a lock on the data that has just been modified is called row-level locking. On the other hand, SQL Server may be set up to lock the database, which is known as database-level locking, found in areas such as backups and restores. The other levels in between are row, page, and table locking, and so you could lock a large resource, depending on the task being performed. This is about as deep as I will take this discussion on locks, so as not to add any confusion or create a problematic situation. Dealing with locks is handled automatically by SQL Server, but it is possible to make locking more efficient by developing an effective understanding of the subject, and then customizing the locks within your transactions.
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Updating Data: Using Transactions
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Now, what if, in the first update query of this chapter, we had made a mistake or an error occurred For example, say we chose the wrong customer, or even worse, omitted the WHERE statement, and therefore all the records were updated. These are unusual errors, but quite possible. More common errors could result from where more than one data modification has to take place and succeed, and the first one succeeds but a subsequent modification fails. By using a transaction, we would have had the chance to correct any mistakes easily, and could then revert to a consistent state. Of course, this next example is nice and simple, but by working through it, the subject of transactions will hopefully become a little easier to understand and appreciate.
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CHAPTER 8 WORKING WITH THE DATA
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Try It Out: Using a Transaction
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1. Make sure Query Editor is running for this first example, which will demonstrate COMMIT TRAN in action. There should be no difference from an UPDATE without any transaction processing, as it will execute and update the data successfully. However, this should prove to be a valuable exercise, as it will also demonstrate the naming of a transaction. Enter the following code: SELECT 'Before',ShareId,ShareDesc,CurrentPrice FROM ShareDetails.Shares WHERE ShareId = 3 BEGIN TRAN ShareUpd UPDATE ShareDetails.Shares SET CurrentPrice = CurrentPrice * 1.1 WHERE ShareId = 3 COMMIT TRAN SELECT 'After',ShareId,ShareDesc,CurrentPrice FROM ShareDetails.Shares WHERE ShareId = 3 Notice in the preceding code that the COMMIT TRAN does not use the name associated with the BEGIN TRAN. The label after the BEGIN TRAN is simply that, a label and performs no functionality. It is therefore not necessary to then link up with a similarly labeled COMMIT TRAN. 2. Execute the code. Figure 8-47 shows the results, which list out the Shares table before and after the transaction.
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Figure 8-47. Updating with transaction label and a COMMIT TRAN 3. We are now going to work through a ROLLBACK TRAN. We will take this one stage at a time so that you fully understand and follow the processes involved. Note in the following code that the WHERE statement has been commented out with --. By having the WHERE statement commented out, hopefully you ll have already guessed that every record in the ShareDetails.Shares table is going to be updated. The example needs you to execute all the code at once, so enter the following code into your Query Editor pane, and then execute it. Note we have three SELECT statements this time before, during, and after the transaction processing. SELECT 'Before',ShareId,ShareDesc,CurrentPrice FROM ShareDetails.Shares -- WHERE ShareId = 3 BEGIN TRAN ShareUpd
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