The main use for external tables is to use them as a row source for loading data into an
actual table in the database. After you create an external table, you can then use a
CREATE TABLE AS SELECT or INSERT INTO ... AS SELECT statement, using the
external table as the source of the SELECT clause.

When you access the external table through a SQL statement, the fields of the external
table can be used just like any other field in a regular table. In particular, you can use
the fields as arguments for any SQL built-in function, PL/SQL function, or Java
function. This lets you manipulate data from the external source. For data
warehousing, you can do more sophisticated transformations in this way than you can
with simple datatype conversions. You can also use this mechanism in data
warehousing to do data cleansing.

While external tables cannot contain a column object, constructor functions can be
used to build a column object from attributes in the external table .

Note: You cannot insert data into external tables or update records in
them; external tables are read only.

外部表加载数据
1. 不能将记录插入到外部表中 , 不能更改外部表中的数据 , 外部表是只读的

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Data Loading with External Tables (172)

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