Sorting Query Results
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, rather than duplicate the entire expression, in the
ORDER BY clause.
- For compound queries (containing set operators
UNION, INTERSECT, MINUS, or UNION ALL), the ORDER BY clause must use positions, 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 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.
