OCP-1Z0-052-V8.02-96题

yinian

96. Which two operations can be performed on an external table? (Choose two.)
A.Create a view on the table.
B.Create an index on the table.
C.Create a synonym on the table.
D.Add a virtual column to the table.
E.Update the table using the UPDATE statement.
F.Delete rows in the table using the DELETE command.
Answer: AC
答案解析：
参考：http://blog.csdn.net/rlhua/article/details/12749863
官方参考：http://docs.oracle.com/cd/E11882_01/server.112/e25494/tables.htm#ADMIN12896



About External Tables
　　
　　
Oracle Database allows you read-only access to data inexternal tables.External tablesare defined as tables that do not reside in the database, and can be in any formatfor which an access driver is provided. By providing the database with metadata describing an external table, the database is able to expose the data in the external table as if it were data residing in a regular database table. The external data can be querieddirectly and in parallel using SQL.
　　You can, for example, select, join, or sort external table data. You can also create views and synonyms for external tables.However, no DML operations (UPDATE,INSERT,orDELETE) are possible, and no indexes can be created, on external tables.
External tables provide a framework to unload the result of an arbitrarySELECTstatement into a platform-independent Oracle-proprietary format that can be used by Oracle Data Pump. External tables provide a valuablemeans for performing basic extraction, transformation, and loading (ETL) tasks that are common for data warehousing.
The means of defining the metadata for external tables is through theCREATE TABLE...ORGANIZATION EXTERNALstatement. This external table definition can be thought of as a view that allows running any SQL query againstexternal data without requiring that the external data first be loaded into the database. An access driver is the actual mechanism used to read the external data in the table. When you use external tables to unload data, the metadata is automatically createdbased on the data types in theSELECTstatement.
Oracle Database provides two access drivers for external tables. The default access driver isORACLE_LOADER, which allows the reading of data from external files using the Oracle loader technology. TheORACLE_LOADERaccessdriver provides data mapping capabilities which are a subset of the control file syntax of SQL*Loader utility. The second access driver,ORACLE_DATAPUMP, lets you unload data—that is, read data from the database and insert it into an externaltable, represented by one or more external files—and then reload it into an Oracle Database.
　　Note:
TheANALYZEstatement is not supported for gathering statistics for external tables. Use theDBMS_STATSpackage instead.