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 character values for the ORDER BY clause, also known as the collation, is specified by the NLS_SORT session parameter. If this parameter is not set, then its default is derived from the NLS_LANGUAGE session parameter. You can change the collation dynamically using the ALTER SESSION SET NLS_SORT statement. You can also apply a specific collation by including the character expressions to be sorted as arguments to the NLSSORT function, with the collation specified in the second parameter.
When character values are compared linguistically for the ORDER BY clause, they are first transformed to collation keys and then compared like RAW values. The collation keys are generated either explicitly as specified in NLSSORT or implicitly using the same method that NLSSORT uses. Both explicitly and implicitly generated collation keys are subject to the same restrictions that are described in "NLSSORT". As a result of these restrictions, two values may compare as linguistically equal if they do not differ in the prefix that was used to produce the collation key, even if they differ in the rest of the value.
See Also:
NLSSORT and Oracle Database Globalization Support Guide for information on the NLS parameters