获取七天以前到现在的数据：
$days = Input::get('days', 7);
$range = \Carbon\Carbon::now()->subDays($days);
$stats = User::where('created_at', '>=', $range)
->groupBy('date')
->orderBy('date', 'DESC')
->get([
DB::raw('Date(created_at) as date'),
DB::raw('COUNT(*) as value')
]);
SELECT
sum(case when `Em
