tailieunhanh - SQL VISUAL QUICKSTART GUIDE- P43

SQL VISUAL QUICKSTART GUIDE- P43:SQL (pronounced es-kyoo-el) is the standard programming language for creating, updating, and retrieving information that is stored in databases. With SQL, you can turn your ordinary questions (“Where do our customers live?”) into statements that your database system can understand (SELECT DISTINCT city, state FROM customers;) | Executing a Transaction Chapter 14 Executing a Transaction To learn how transactions work you need to learn a few terms Commit. Committing a transaction makes all data modifications performed since the start of the transaction a permanent part of the database. After a transaction is committed all changes made by the transaction become visible to other users and are guaranteed to be permanent if a crash or other failure occurs. Roll back. Rolling back a transaction retracts any of the changes resulting from the SQL statements in the transaction. After a transaction is rolled back the affected data are left unchanged as though the SQL statements in the transaction were never executed. Transaction log. The transaction log file or just log is a serial record of all modifications that have occurred in a database via transactions. The transaction log records the start of each transaction the changes to the data and enough information to undo or redo the changes made by the transaction if necessary later . The log grows continually as transactions occur in the database. Although it s the DBMS s responsibility to ensure the physical integrity of each transaction it s your responsibility to start and end transactions at points that enforce the logical consistency of the data according to the rules of your organization or business. A transaction should contain only the SQL statements necessary to make a consistent change no more and no fewer. Data in all referenced tables must be in a consistent state before the transaction begins and after it ends. When you re designing and executing transactions some important considerations are Transaction-related SQL statements modify data so your database administrator might need to grant you permission to run them. Transaction processing applies to statements that change data or database objects INSERT UPDATE DELETE CREATE ALTER DROP the list varies by DBMS . For production databases every such statement should be executed as part of a