关于 Laravel5.8 子查询的问题

$ads = DB::table('ub_ads')->selectRaw("coverimage, is_publish, type,
            merchant_id, title, content, remarks, url, created_at, updated_at")
            ->where('position', 'merchant-info')->get();
$result = DB::table('ub_merchants as m')
            ->joinSub($ads, 'ads', function (Builder $join) {
            $join->on('m.id', '=', 'ads.merchant_id');
        })->get();
        dd($result);Copy