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Transaction Processing
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I Reassign-a-customer. When a customer is reassigned from one salesperson to another, the program should (a) update the CUSTOMERS table to reflect the change, (b) update the ORDERS table to show the new salesperson for all orders placed by the customer, (c) update the SALESREPS table, reducing the quota for the salesperson losing the customer, and (d) update the SALESREPS table, raising the quota for the salesperson gaining the customer. In each of these cases, a sequence of four or five actions, where each action consists of a separate SQL statement, is required to handle the single logical transaction. The transaction concept is critical for programs that update a database because it ensures the integrity of the database. A SQL-based DBMS makes this commitment about the statements in a transaction: The statements in a transaction will be executed as an atomic unit of work in the database. Either all of the statements will be executed successfully, or none of the statements will be executed. The DBMS is responsible for keeping this commitment even if the application program aborts or a hardware failure occurs in the middle of the transaction, as shown in Figure 12-1. In each case, the DBMS must make sure that when failure recovery is complete, the database never reflects a partial transaction.
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COMMIT and ROLLBACK
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SQL supports database transactions through two SQL transaction-processing statements, shown in Figure 12-2: I COMMIT. The COMMIT statement signals the successful end of a transaction. It tells the DBMS that the transaction is now complete; all of the statements that comprise the transaction have been executed, and the database is self-consistent. I ROLLBACK. The ROLLBACK statement signals the unsuccessful end of a transaction. It tells the DBMS that the user does not want to complete the transaction; instead, the DBMS should back out any changes made to the database during the transaction. In effect, the DBMS restores the database to its state before the transaction began. The COMMIT and ROLLBACK statements are executable SQL statements, just like SELECT, INSERT, and UPDATE. Here is an example of a successful update transaction that changes the quantity and amount of an order and adjusts the totals for the product, salesperson, and office associated with the order. A change like this would typically be handled by a forms-based change order program, which would use programmatic SQL to execute the statements shown here.
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12:
Transaction Processing
Change the quantity on order number 113051 from 4 to 10, which raises its amount from $1458 to $3550. The order is for QSA-XK47 reducers and was placed with Larry Fitch (employee number 108) who works in Los Angeles (office number 21).
UPDATE ORDERS SET QTY = 10, AMOUNT = 3550.00 WHERE ORDER_NR = 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 = 'QSA' PRODUCT_ID = 'XK47'
UPDATING DATA
. . . confirm the change one last time with the customer . . .
COMMIT WORK
Here is the same transaction, but this time assume that the user makes an error entering the product number. To correct the error, the transaction is rolled back, so that it can be reentered correctly: Change the quantity on order number 113051 from 4 to 10, which raises its amount from $1458 to $3550. The order is for QAS-XK47 reducers and was placed with Larry Fitch (employee number 108), who works in Los Angeles (office number 21).
UPDATE ORDERS SET QTY = 10, AMOUNT = 3550.00 WHERE ORDER_NR = 113051 UPDATE SALESREPS SET SALES = SALES - 1458.00 + 3550.00 WHERE EMPL_NUM = 108
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