从isp\u acknows i left join pdfreads p on i.id=p.ips\u acknow\u id中选择distinct i.filename“filename”，p.ips\u acknow\u id“isp acknow id”
我也可以通过连接来实现这一点，但是我需要这种格式来提高性能。

Note:
1. isp_acknows table
        Schema::create('isp_acknows', function (Blueprint $table) {
            $table->id();
            $table->timestamps();
            $table->unsignedInteger('user_id');
            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
            $table->string('filename')->nullable();
            $table->string('filedisplayname')->nullable();
            $table->string('privilege')->nullable();
            $table->string('fileversion')->nullable();
            $table->string('remarks')->nullable();
            $table->softDeletes();
        });

2. pdfreads Table

        Schema::create('pdfreads', function (Blueprint $table) {
            $table->id();
            $table->unsignedInteger('user_id');
            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
            $table->unsignedInteger('ips_acknow_id');
            $table->unique(['user_id','ips_acknow_id']);
            $table->timestamps();