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Transactions represent real-world events such as bank transactions, airline reservations, remittance of funds, and so forth. The purpose of transaction design is to define and document the high-level characteristics of transactions required on the database system, including the following: Data to be used by the transaction Functional characteristics of the transaction Output of the transaction Importance to users Expected rate of usage
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CHAPTER 8 s UNDERSTANDING TRANSACTIONS
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There are three main types of transactions: Retrieval transactions: Retrieves data from display on the screen Update transactions: Inserts new records, deletes old records, or modifies existing records in the database Mixed transactions: Involves both retrieval and updating of data
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Transaction State
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In the absence of failures, all transactions complete successfully. However, a transaction may not always complete its execution successfully. Such a transaction is termed aborted. A transaction that completes its execution successfully is said to be committed. Figure 8-1 shows that if a transaction has been partially committed, it will be committed but only if it has not failed; and if the transaction has failed, it will be aborted.
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Figure 8-1. States of a transaction
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Specifying Transaction Boundaries
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SQL Server transaction boundaries help you to identify when SQL Server transactions start and end by using API functions and methods: Transact-SQL statements: Use the BEGIN TRANSACTION, COMMIT TRANSACTION, COMMIT WORK, ROLLBACK TRANSACTION, ROLLBACK WORK, and SET IMPLICIT_TRANSACTIONS statements to delineate transactions. These are primarily used in DB-Library applications and in T-SQL scripts, such as the scripts that are run using the osql command-prompt utility. API functions and methods: Database APIs such as ODBC, OLE DB, ADO, and the .NET Framework SQLClient namespace contain functions or methods used to delineate transactions. These are the primary mechanisms used to control transactions in a database engine application.
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CHAPTER 8 s UNDERSTANDING TRANSACTIONS
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Each transaction must be managed by only one of these methods. Using both methods on the same transaction can lead to undefined results. For example, you should not start a transaction using the ODBC API functions, and then use the T-SQL COMMIT statement to complete the transaction. This would not notify the SQL Server ODBC driver that the transaction was committed. In this case, use the ODBC SQLEndTran function to end the transaction.
T-SQL Statements Allowed in a Transaction
You can use all T-SQL statements in a transaction, except for the following statements: ALTER DATABASE, RECONFIGURE, BACKUP, RESTORE, CREATE DATABASE, UPDATE STATISTICS, and DROP DATABASE. Also, you cannot use sp_dboption to set database options or use any system procedures that modify the master database inside explicit or implicit transactions.
Local Transactions in SQL Server 2005
All database engines are supposed to provide built-in support for transactions. Transactions that are restricted to only a single resource or database are known as local transactions. Local transactions can be in one of the following four transaction modes: Autocommit Transactions Autocommit mode is the default transaction management mode of SQL Server. Every T-SQL statement is committed or rolled back when it is completed. If a statement completes successfully, it is committed; if it encounters any errors, it is bound to roll back. A SQL Server connection operates in autocommit mode whenever this default mode has not been overridden by any type transactions. Explicit Transactions Explicit transactions are those in which you explicitly control when the transaction begins and when it ends. Prior to SQL Server 2000, explicit transactions were also called user-defined or user-specified transactions. T-SQL scripts for this mode use the BEGIN TRANSACTION, COMMIT TRANSACTION, and ROLLBACK TRANSACTION statements. Explicit transaction mode lasts only for the duration of the transaction. When the transaction ends, the connection returns to the transaction mode it was in before the explicit transaction was started. Implicit Transactions When you connect to a database using SQL Server Management Studio Express and execute a DML query, the changes are automatically saved. This occurs because, by default, the connection is in autocommit transaction mode. If you want no changes to be committed unless you explicitly indicate so, you need to set the connection to implicit transaction mode. You can set the database connection to implicit transaction mode by using SET IMPLICIT _TRANSACTIONS ON|OFF. After implicit transaction mode has been set to ON for a connection, SQL Server automatically starts a transaction when it first executes any of the following statements: ALTER TABLE, CREATE, DELETE, DROP, FETCH, GRANT, INSERT, OPEN, REVOKE, SELECT, TRUNCATE TABLE, and UPDATE.
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