Which of the following statements about external tables is true?

A.
They can have constraints or triggers.

B.
They cannot be written to with DML commands.

C.
They can have indexes.

D.
They cannot be used in joins, views, and subqueries.

Explanation:
An external table is a read-only table stored outside a database in flat files. The metadata for an
external table is not created at the time of table creation. However, the CREATE TABLE statement is
used to create metadata. External tables are used to load and unload Oracle data as flat files. The
following can be used to query data stored in an external table:
SQL statements
PL/SQL
JAVA
Indexes cannot be created for an external table. The following are the two types of access drivers
provided by an Oracle server for an external table:ORACLE_LOADER (Loader access driver): It is used
to read data from external files by using Oracle Loader technology. The data format should be
interpreted by SQL*Loader utility.ORACLE_INTERNAL (Import/export access driver): It is used to
import and export data. The import and export operations are platform independent.
Some additional characteristics of external tables are mentioned below:
They can be queried in a similar manner as internal tables.
They can be used in joins, views, and subqueries.
They cannot have indexes, constraints, or, triggers.
They cannot be written to with DML commands.
Note: It is possible to write to an external table with Data Pump.