laravel framework implements search function code analysis

<form action="/backend/auditList" method="get">
  <table class="search_tab">
    <tr>
      <th width="120">选择分类:</th>
      <td>
        <select name="class" >
          <option value="">全部</option>
          @foreach($category as $c)
            <option value="{{$c->id}}">{{$c->class_name}}</option>
          @endforeach
        </select>
      </td>
      <th width="70">文章标题:</th>
      <!--查询关键词-->
      <td><input type="text" name="keywords" placeholder="文章标题"></td>
      <td><input type="submit" name="sub" value="查询"></td>
    </tr>
  </table>
</form>

public function article_list(){
  //echo 'zoule';exit; 测试表单是否走进方法中 大家随意写
  $shownum = 1;
  if(array_key_exists('class',$_GET)||array_key_exists('keywords',$_GET)){
   //echo '111'; 
   if($_GET['class']){
   //Article模型 leftJoin表连接 查询根据俩个表里的这些字段来执行
     $postdata = Article::leftJoin('category', function($join) {
      $join->on('article.class_id', '=', 'category.id');
     })->select(['article.id','category.class_name','article.status','article.title_editing','article.update_time'])->where('article.class_id','=',$_GET['class'])->orderBy('release_time','desc')->paginate($shownum);
   }elseif($_GET['keywords']){
     $postdata = Article::leftJoin('category', function($join) {
      $join->on('article.class_id', '=', 'category.id');
     })->select(['article.id','category.class_name','article.status','article.title_editing','article.update_time'])->where('article.title_editing','=',$_GET['keywords'])->orderBy('release_time','desc')->paginate($shownum);
    
   }else{
     $postdata = Article::leftJoin('category', function($join) {
      $join->on('article.class_id', '=', 'category.id');
     })->select(['article.id','category.class_name','article.status','article.title_editing','article.update_time'])->orderBy('release_time','desc')->paginate($shownum);
   }
  }else{
   //echo '2222';
   $postdata = Article::leftJoin('category', function($join) {
     $join->on('article.class_id', '=', 'category.id');
   })->select(['article.id','category.class_name','article.status','article.title_editing','article.update_time'])->orderBy('release_time','desc')->paginate($shownum);
  }
  //分类id不是父id
  $category = DB::table('category')->where('parent_id','!=','0')->get();
  //渲染页面 传递 参数
  return view('backend.article_list',['postdata'=>$postdata,'shownum'=>$shownum,'category'=>$category]);
}