Sometimes a plugin comes along which changes your life. Fast SQL Layer from Pablo T. Carreira is just such a plugin.
It allows you to execute arbitrary SQL queries against your underlying database, meaning that views and queries can now live in your QGIS project where they belong, instead of cluttering up the poor unsuspecting PostGIS database that’s having to do all the hard work.
To install it (on Windows at least), add the QGIS Contributed repository to your QGIS repository list (http://pyqgis.org/repo/contributed) and you’ll be writing queries in no time at all. Amazing.
Data-driven economics 