问一下大家，Laravel 里数据查询 如何使用子查询

文档里有，类似这样：

$sub = SyncLog::select('insert_num', 'created_at', 'end_time')->where('status', '=', 1)->orderBy('id', 'desc');

$dailyResults = DB::table( DB::raw("({$sub->toSql()}) as sub") )
                    ->mergeBindings($sub->getQuery())
                    ->select(DB::raw('end_time, sum(insert_num) as total'))
                    ->groupBy('end_time')
                    ->orderBy('end_time', 'desc')
                    ->limit($days)
                    ->get();