EllisLab/CodeIgniter — pull request #2781 by tufanbarisyildirim (2013-12-19 17:51:30+00:00)
Title: EXISTS / NOT EXISTS clause support for  DB_query_builder.

I need a query contains EXISTS () to check permission or non-completed relations etc. So , I have just added EXISTS support to query_builder to be able to use as ;

```
->order_by('post_date','DESC')
->where('EXISTS','(SELECT * FROM blog_privacy WHERE blog_privacy.post_id = blogs.post_id)',false)
```