Question: How to export Oracle DB data using SQL Developer? I need all data, tables, constraints, structure and so on.

Answer: 
In SQL Developer, from the top menu choose Tools > Data Export. This launches the Data Export wizard. It's pretty straightforward from there. 

There is a tutorial on the OTN site. [Find it here](http://www.oracle.com/webfolder/technetwork/tutorials/obe/db/sqldev/r30/SQLdev3.0_Import_Export/sqldev3.0_import_export.htm).
If, at some point, you only need to export a single result set, just right click "on the data" (any column of any row) there you will find an export option.
The wizard exports the complete result set regardless of what you selected