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UPDATE ORDERS SET QTY = 10, AMOUNT = 3550.00 WHERE ORDER_NUM = 113051; UPDATE SALESREPS SET SALES = SALES - 1458.00 + 3550.00 WHERE EMPL_NUM = 108; 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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Figure 12-4 shows typical transactions that illustrate common conditions.
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Tr a n s a c t i o n P r o c e s s i n g
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 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.
Transactions: Behind the Scenes*
The all-or-nothing commitment that a DBMS makes for the statements in a transaction seems almost like magic to a new SQL user. How can the DBMS possibly back out the changes made to a database, especially if a system failure occurs during the middle of a transaction The actual techniques used by brands of DBMS vary, but almost all of them are based on a transaction log, as shown in Figure 12-5. Although the term log is commonly used, some DBMS products use more elaborate mechanisms to store database change information needed for recovery. Oracle, for example, stores database changes in special database segments. Here is how the transaction log works, in simplified, conceptual form. When a user executes a SQL statement that modifies the database, the DBMS automatically writes a record in the transaction log showing two copies of each row affected by the statement. One copy shows the row before the change, and the other copy shows the row after the change. Only after the log is written does the DBMS actually modify the row on the disk. If the user subsequently executes a COMMIT statement, the end-of-transaction is noted in the transaction log. If the user executes a ROLLBACK statement, the DBMS examines the log to find the before images of the rows that have been modified since the transaction began. Using these images, the DBMS restores the rows to their earlier state, effectively backing out all database changes made during the transaction. In older database systems, when a system failure occurred, the system operator typically recovered the database by running a special recovery utility supplied with the DBMS. In newer database systems, the recovery operation normally runs automatically at the first available opportunity. The recovery utility examines the end of the transaction log, looking for transactions that were not committed before the failure. The utility rolls back each of these incomplete transactions, so that only committed transactions are reflected in the database; transactions in process at the time of the failure have been rolled back. The use of a transaction log obviously imposes an overhead on updates to the database. In practice, the mainstream commercial DBMS products minimize this overhead by using
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