What Are External Tables
External tables are new to Oracle9i. This feature enables computer files to be accessed while still outside the Oracle database. These files that become relational tables are read-only, cannot have indexes (no ROWID), and are designed for data loading, allowing the load process to take full advantage of the SQL language. For example, when loading, it might be desirable to make changes to it upon the load (such as using a DECODE function to change data values or concatenating fields together). Using SQL*Loader, you would have to load a table then use SQL syntax to perform the desirable changes. Using external tables, this is accomplished in a single step.
External Table Syntax
External table definitions resemble SQL*Loader ...