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SQL: The Complete Reference
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UPDATE OFFICES SET SALES = SALES - 1458.00 + 3550.00 WHERE OFFICE = 21 UPDATE SET WHERE AND PRODUCTS QTY_ON_HAND = QTY_ON_HAND + 4 - 10 MFR_ID = 'QAS' PRODUCT_ID = 'XK47'
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The ANSI/ISO Transaction Model
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The ANSI/ISO SQL standard defines a SQL transaction model and the roles of the COMMIT and ROLLBACK statements. Most, but not all, commercial SQL products use this transaction model, which is based on the transaction support in the early releases of DB2. The standard specifies that a SQL transaction automatically begins with the first SQL statement executed by a user or a program. The transaction continues through subsequent SQL statements until it ends in one of four ways: I COMMIT. A COMMIT statement ends the transaction successfully, making its database changes permanent. A new transaction begins immediately after the COMMIT statement. I ROLLBACK. A ROLLBACK statement aborts the transaction, backing out its database changes. A new transaction begins immediately after the ROLLBACK statement. I Successful program termination. For programmatic SQL, successful program termination also ends the transaction successfully, just as if a COMMIT statement had been executed. Because the program is finished, there is no new transaction to begin. I Abnormal program termination. For programmatic SQL, abnormal program termination also aborts the transaction, just as if a ROLLBACK statement had been executed. Because the program is finished, there is no new transaction to begin. Figure 12-3 shows typical transactions that illustrate these four conditions. Note that the user or program is always in a transaction under the ANSI/ISO transaction model. No explicit action is required to begin a transaction; it begins automatically with the first SQL statement or immediately after the preceding transaction ends. Recall that the ANSI/ISO SQL standard is primarily focused on a programmatic SQL language for use in application programs. Transactions play an important role in programmatic SQL, because even a simple application program often needs to carry out
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Figure 12-3.
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Committed and rolled back transactions
a sequence of two or three SQL statements to accomplish its task. Because users can change their minds and other conditions can occur (such as being out of stock on a product that a customer wants to order), an application program must be able to proceed partway through a transaction and then choose to abort or continue. The COMMIT and ROLLBACK statements provide precisely this capability. The COMMIT and ROLLBACK statements can also be used in interactive SQL, but in practice, they are rarely seen in this context. Interactive SQL is generally used for database queries; updates are less common, and multistatement updates are almost never performed by typing the statements into an interactive SQL facility. As a result, transactions are typically a minor concern in interactive SQL. In fact, many interactive SQL products default to an autocommit mode, where a COMMIT statement is automatically executed after each SQL statement typed by the user. This effectively makes each interactive SQL statement its own transaction.
SQL: The Complete Reference
Other Transaction Models
A few commercial SQL products depart from the ANSI/ISO transaction model to provide additional transaction-processing capability for their users. The Sybase DBMS, which is designed for online transaction-processing applications, is one example. SQL Server, which was derived from the Sybase product, also uses the Sybase transaction model. The Transact-SQL dialect used by Sybase includes four transaction-processing statements: I BEGIN TRANSACTION. The BEGIN TRANSACTION statement signals the beginning of a transaction. Unlike the ANSI/ISO transaction model, which implicitly begins a new transaction when the previous one ends, Sybase requires an explicit statement to start a transaction. I COMMIT TRANSACTION. Signals the successful end of a transaction. As in the ANSI/ISO model, all changes made to the database during the transaction become permanent. However, a new transaction is not automatically started. I SAVE TRANSACTION. Establishes a savepoint in the middle of a transaction. Sybase saves the state of the database at the current point in the transaction and assigns the saved state a savepoint name, specified in the statement. I ROLLBACK TRANSACTION. Has two roles. If a savepoint is named in the ROLLBACK statement, Sybase backs out the database changes made since the savepoint, effectively rolling the transaction back to the point where the SAVE TRANSACTION statement was executed. If no savepoint is named, the ROLLBACK statement backs out all database changes made since the BEGIN TRANSACTION statement. The Sybase savepoint mechanism is especially useful in complex transactions involving many statements, as shown in Figure 12-4. The application program in the figure periodically saves its status as the transaction progresses, establishing two named savepoints. If problems develop later during the transaction, the application program does not have to abort the entire transaction. Instead, it can roll the transaction back to any of its savepoints and proceed from there. All of the statements executed before the savepoint remain in effect; those executed since the savepoint are backed out by the rollback operation. Note that the entire transaction is still the logical unit of work for Sybase, as it is for the ANSI/ISO model. If a system or hardware failure occurs in the middle of a transaction, for example, the entire transaction is backed out of the database. Thus, savepoints are a convenience for the application program, but not a fundamental change to the ANSI/ISO transaction model. The explicit use of a BEGIN TRANSACTION statement is, however, a significant departure from the ANSI/ISO model. SQL statements that are executed outside a transaction (that is, statements that do not appear between a BEGIN/COMMIT or a
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