Stuck on Random & Unique in Query Builder

Hi - I'm been at Laravel for a couple of weeks now and am getting the hang of it but am struggling with a kind of complex query I need to build. Essentially I have three tables: polaroids (a list of files), tags (a list of keywords) and polaroid_tag (a pivot table for the two). Essentially a user selects an array of tags and passes the tag id number array through the URL parameter and I need to query it to:

1. Get the keyword text (tag) from associated with the id.
2. Count the number of occurrences of each keyword in the pivot table
3. Pick a single polaroid row to display as an example.

This part all works - but I would like selection of the single polaroid to be random and distinct from any other it selects. This is what I have so far:

public function getIndex()
        {
            $getTags = Request::query('tag');
            $tagTable = DB::table('tags')
                                 ->join('polaroid_tag', 'tags.id', '=', 'polaroid_tag.tag_id')
                                 ->join('polaroids', 'polaroids.id', '=', 'polaroid_tag.polaroid_id')
                                 ->select(DB::raw('count(*) as tag_count, tag, tag_id as id, filename, polaroid_id'))
                                 ->whereIn('tags.id', $getTags)
                                 ->groupBy('tag')
                                 ->get();
            $this->layout = View::make('story.close',compact('tagTable'));
        }

When testing in my phpMyAdmin using

ORDER BY RAND( )

in my INNER JOIN generated random selections, but I don't know how to mimic this in Laravel Query Builder, also it would not make them necessarily distinct if 3 tags were being displayed.

Any guidance would be appreciated! Thanks.