Use the ORDER BY clause to order the rows selected by a query. Sorting by position is useful in the following cases:
To order by a lengthy select list expression, you can specify its position in the ORDER BY clause rather than duplicate the entire expression.
For compound queries containing set operators UNION, INTERSECT, MINUS, or UNION ALL, the ORDER BY clause must specify positions or aliases rather than explicit expressions. Also, the ORDER BY clause can appear only in the last component query. The ORDER BY clause orders all rows returned by the entire compound query.
The mechanism by which Oracle Database sorts values for the ORDER BY clause is specified either explicitly by the NLS_SORT initialization parameter or implicitly by the NLS_LANGUAGE initialization parameter. You can change the sort mechanism dynamically from one linguistic sort sequence to another using the ALTER SESSION statement. You can also specify a specific sort sequence for a single query by using the NLSSORT function with the NLS_SORT parameter in the ORDER BY clause.
See Also:
NLSSORT and Oracle Database Globalization Support Guide for information on the NLS parameters