how to export data into excel from oracle sql developer 3.0.04

• Retrieve the data you need with the select statement.
• Select all the required data.
• Right click and then you can find the export button. 
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• Select the xls for excel sheet and then follow the instructions.
• The path where you have saved the sql file will give you the exported table data.

How to select 'n' number of rows in oracle?

Use this query,

select * from table_name where rownum < 'n';

table_name is your table name.
'n' is number of rows you want to retrive.