Working with Transact-SQL in Visual Studio .NET

Printing PDF 417 in Visual Studio .NET Working with Transact-SQL

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Working with Transact-SQL
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Lesson 4: Working with Transactions
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When modifying data, it s important to ensure that only correct data gets written to the database. By controlling transactions and handling errors, developers can make sure that if problems do occur when modifying data, incorrect data can be selectively kept out of the database.
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After this lesson, you will be able to:
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Begin and commit or roll back transactions. Programmatically handle errors.
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Estimated lesson time: 20 minutes
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Beginning and Committing or Rolling Back Transactions
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When modifying data in the database, one of the most important things developers need to consider is how best to keep the data in a consistent state. Consistent state means that all data in the database should be correct at all times incorrect data must be removed or, better yet, not inserted at all. Transactions are the primary mechanism by which you can programmatically enforce data consistency. When you begin a transaction, any data changes you make are, by default, visible only to your connection. Other connections reading the data cannot see the changes you make and have to wait until you either commit the transaction thereby saving the changes to the database or roll it back, thereby removing the changes and restoring the data to the state it was in before the transaction started. The basic process to use when working with transactions is as follows: 1. Start transactions by using the BEGIN TRANSACTION command. 2. After you start a transaction by using BEGIN TRANSACTION, the transaction will encompass all data modifications made by your connection, including inserts, updates, and deletes. 3. The transaction ends only when you either commit it or roll it back. You can commit a transaction, saving the changes, by using the COMMIT TRANSACTION command. You roll back a transaction by using the ROLLBACK TRANSACTION command. If at any time after the start of the transaction you detect that a problem has occurred, you can use ROLLBACK TRANSACTION to return the data to its original state.
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Lesson 4: Working with Transactions
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BEST PRACTICES
Use transactions for testing
Transactions can be very useful if you re testing code that modifies data in the database. Begin a transaction before running your code, and then roll back the transaction when you re done testing. Your data will be in the same state it was in when you started.
Programmatically Handle Errors
The ability to begin transactions and selectively commit them or roll them back is not quite enough to be able to effectively deal with problems when they occur. The other necessary component is the ability to programmatically detect and handle errors. You perform error checking in Transact-SQL by using the TRY and CATCH control-offlow statements. TRY defines a block within which you place code that might cause an error. If any of the code in the block causes an error, processing immediately halts, and the code in the CATCH block is run. The following code shows the basic TRY/ CATCH format:
BEGIN TRY --Put error-prone code here END TRY BEGIN CATCH --Put error handling code here END CATCH
Within the CATCH block, you can determine what caused the error and get information about the error by using the Transact-SQL error handling system functions. The m o s t c o m m o n l y u s e d o f t h e s e f u n c t i o n s a re E R RO R _ N U M B E R a n d ERROR_MESSAGE, which return the error number for the error and the text description for the error, respectively. Other available functions include ERROR_LINE, ERROR_SEVERITY, and ERROR_STATE. By using these functions in the CATCH block, you can determine whether you need to use ROLLBACK to roll back your transaction.
Quick Check
Into which block should you place code that might cause an error Code that might cause an error should be put into the TRY block.
Quick Check Answer
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Working with Transact-SQL
PRACTICE
Seeing the Effect of Transactions
In this practice, you will see how transactions affect other connections. 1. If necessary, open SSMS and connect to your SQL Server. 2. Open a new query window and select AdventureWorks as the active database. 3. Type the following query and execute it:
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