You define a descending-key index by using the keyword DESC when you create the index.
When DESC is specified, the columns in the index are sorted in descending order according
to the database character set. In prior versions of Oracle, the DESC keyword was ignored.
Oracle 8i treats descending-key indexes as function-based indexes. However, you do not
need the QUERY REWRITE privileges to create a DESC index. Oracle does need statistics
on the DESC index and its base table, so they must be analyzed at least once or else
the DESC index will not be used. Reverse or bitmap indexes cannot use the DESC option.
Below is the correct syntax used to create a descending index.
CREATE INDEX myindx_desc ON mytab (emplid DESC);
In practice, there is very little use to descending indexes. Every benefit of descending indexes is also present in the default ascending indexes. Descending indexes are almost never used in real life situations.