barcode generator in vb.net codeproject Committing a Transaction in Software

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Committing a Transaction
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Once all the statements have been executed in a transaction, the transaction must be terminated. The preferable type of termination is one that commits all the changes to the database. After all, why try to make changes if you don t want to commit them To explicitly commit the changes and end the transaction, you must use the COMMIT statement, as shown in the following syntax: COMMIT [ WORK ] [ AND [ NO ] CHAIN ]
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SQL: A Beginner s Guide
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At its most basic, the COMMIT statement requires only the COMMIT keyword. All other statement elements are optional. If you want, you can include the WORK keyword, which is simply a carryover from earlier versions of SQL. In other words, COMMIT and COMMIT WORK perform the same function. The only reason to use the WORK keyword is if your SQL implementation requires it. The next optional element in the COMMIT statement is the AND CHAIN clause, which is not widely supported in current SQL implementations. The clause tells the system to start a new transaction as soon as the current transaction ends. The new transaction uses the same transaction modes as the current transaction. If you use the AND CHAIN option, you do not need to use the SET TRANSACTION or the START TRANSACTION statements for the next transaction unless you want to specify different modes. Rather than specify AND CHAIN in your COMMIT statement, you can specify AND NO CHAIN, which tells your system not to start a new transaction based on the settings of the current transaction. If AND NO CHAIN is specified, a new transaction will not be initiated automatically when the current transaction is terminated. You must start a new transaction by using an implicit method or explicit method. If neither the AND CHAIN clause nor the AND NO CHAIN clause is specified, AND NO CHAIN is assumed. In all likelihood, your commit statement will look like the one in the following example:
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COMMIT;
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As you can see, the COMMIT keyword is the only required element. However, if you want a new transaction to be initiated after the current one, you should use the following COMMIT statement:
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COMMIT AND CHAIN;
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If you don t want a new transaction to be initiated, do not include the AND CHAIN clause.
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Rolling Back a Transaction
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Although the goal of any transaction is to commit the changes made by the statements in that transaction, there will no doubt be times when you want to roll back those changes. To be able to control these rollbacks, you must use a ROLLBACK statement to undo changes and terminate the transaction, or to undo changes back to a specified savepoint. The following syntax shows the various elements that can be included in a ROLLBACK statement: ROLLBACK [ WORK ] [ AND [ NO ] CHAIN ] [ TO SAVEPOINT <savepoint name> ] The first line of syntax is very similar to the COMMIT statement. You must specify the ROLLBACK keyword. In addition, you can specify WORK, AND CHAIN, or AND NO CHAIN, all of which work the same way they did in the COMMIT statement, with AND NO CHAIN once again being the default.
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16:
Managing SQL Transactions
However, the ROLLBACK statement, unlike the COMMIT statement, includes the optional TO SAVEPOINT clause. The TO SAVEPOINT clause specifies a savepoint that is used if changes have to be rolled back. This applies to any changes made after the specified savepoint. If you include the TO SAVEPOINT clause in your ROLLBACK statement, the transaction will be rolled back to the savepoint, but it will not be terminated. If the TO SAVEPOINT clause is not included, all changes are rolled back and the transaction is terminated. The most basic type of ROLLBACK statement is one that includes no optional elements, as in the following example:
ROLLBACK;
You could have included the WORK keyword and the AND NO CHAIN clause, and the statement would have performed the same function. If you want a new transaction to be initiated when the current transaction is terminated, you must specify the AND CHAIN clause. Keep in mind, however, that you cannot specify the AND CHAIN clause and the TO SAVEPOINT clause because AND CHAIN relies on the transaction being terminated in order to start a new transaction. If you do specify the TO SAVEPOINT clause, you must include the name of the savepoint. For example, the following ROLLBACK statement specifies the SECTION_1 savepoint:
ROLLBACK TO SAVEPOINT SECTION_1;
If this statement is executed, all changes that occurred after the SECTION_1 savepoint was created are rolled back to the state the database was in when the SAVEPOINT statement was executed. Even if other savepoints were created after the SECTION_1 savepoint, changes are still rolled back to SECTION_1.
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