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Here is an example of a START TRANSACTION statement that sets an isolation level of READ UNCOMMITTED, an access level of READ ONLY, and a diagnostics size of 5:
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START TRANSACTION ISOLATION LEVEL READ UNCOMMITTED, READ ONLY, DIAGNOSTICS SIZE 5;
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The SAVEPOINT and RELEASE SAVEPOINT Statements
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As stated earlier, the SAVEPOINT statement establishes a point within a transaction to which the transaction can be rolled back using a subsequent ROLLBACK statement. The syntax is quite simple, with the only parameter being a unique name for the savepoint within the transaction:
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SAVEPOINT savepoint-name;
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The obvious advantage of using a savepoint is the ability to roll back part of a transaction when a minor and potentially recoverable error condition is encountered. The rightmost transaction in Figure 12-1 shows such an example. In most implementations, transactions can have as many savepoints as necessary, provided each is given a unique name within the transaction. For example, an order-entry application might create a savepoint after each line item is entered on the order. Should the addition of a new line item exceed the credit limit of the customer, the application can roll back to the savepoint just before the current line item. The rollback would reverse the line item that caused the problem, allowing the application to present an error message to the person entering the order, and then to proceed from that point (perhaps with a less expensive item that would not exceed the limit). The disadvantage of savepoints is that they potentially require a lot of resources (storage and/or memory). While termination of the transaction releases all the savepoints automatically, sometimes it is wise to explicitly release savepoints that are no longer necessary. This can be done with the RELEASE SAVEPOINT statement, which has syntax that is equally simple:
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RELEASE SAVEPOINT savepoint-name;
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The COMMIT and ROLLBACK Statements
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SQL supports two SQL transaction-processing statements for explicitly ending transactions, shown in Figure 12-3. Options include the following: WORK This keyword has no effect, but is included in the standard for compatibility with some SQL implementations that require it. AND [NO] CHAIN Specifies whether a new transaction is to be automatically started with the same properties as the one that just ended. As of this writing, this option is not supported by Oracle, SQL Server, DB2 UDB, or MySQL. TO SAVEPOINT For the ROLLBACK statement only, this option specifies rollback to a savepoint that was previously created within the transaction instead of to the beginning of the transaction.
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12:
Tr a n s a c t i o n P r o c e s s i n g
COMMIT WORK AND [NO] CHAIN
ROLLBACK WORK AND [NO] CHAIN TO SAVEPOINT savepoint-name
FIGURE 12-3
The COMMIT and ROLLBACK statement syntax diagrams
The COMMIT and ROLLBACK statements are executable SQL statements, just like SELECT, INSERT, UPDATE, and DELETE. 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. 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 (of ce number 21).
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 = 'QSA' PRODUCT_ID = 'XK47';
PART III
. . . con rm 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 (of ce number 21).
Part III:
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