本文深入探讨了Oracle外部表的概念，包括其如何在外部源中访问数据，以及如何通过DDL创建元数据。强调了外部表的只读性质和无法进行DML操作的特点，同时解释了DDL中描述列类型和映射外部数据到Oracle数据列的过程。
External tables access data in external sources as if it were in a table in the database.
You can connect to the database and create metadata for the external table using DDL.
The DDL for an external table consists of two parts: one part that describes the Oracle
column types, and another part (the access parameters) that describes the mapping of
describe how data is stored in the external source. Instead, it describes how the
external table layer needs to present the data to the server. It is the responsibility of the
access driver and the external table layer to do the necessary transformations required
on the data in the datafile so that it matches the external table definition.
External tables are read only; therefore, no DML operations are possible, and no index
You can connect to the database and create metadata for the external table using DDL.
The DDL for an external table consists of two parts: one part that describes the Oracle
column types, and another part (the access parameters) that describes the mapping of
the external data to the Oracle data columns.
外部表访问数据在外部源尽管它在数据库的表中。你可以连接数据库并且创建外部表的元
数据使用DDL语句。外部表的DDL由两部份组成：一部份描述ORACLE的列类型，另一部
份（访问参数）它描述外部数据与ORACLE数据列的对应关系。
describe how data is stored in the external source. Instead, it describes how the
external table layer needs to present the data to the server. It is the responsibility of the
access driver and the external table layer to do the necessary transformations required
on the data in the datafile so that it matches the external table definition.
External tables are read only; therefore, no DML operations are possible, and no index
can be created on them.
一个外部表不描述任何存储在数据库中的数据。它也不描述数据是如何存储在外部源。取代的是，
它描述了外部表层怎样需要去描述数据给服务器。它是由访问的驱动负责的并且外部表层做要求
需要转换的数据在数据文件上，这样它能满足外部表的定义。
外部表是只读的；所以，没有DML操作可能操作，以及没有索引可以创建在上面。
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