这取决于您是想在之后使用用户，还是只检查是否存在一个用户。

如果用户对象存在，你想使用它:

$user = User::where('email', '=', Input::get('email'))->first();
if ($user === null) {
   // user doesn't exist
}

如果你只是想检查一下

if (User::where('email', '=', Input::get('email'))->count() > 0) {
   // user found
}

或者更好

if (User::where('email', '=', Input::get('email'))->exists()) {
   // user found
}

如果你想插入一个唯一的记录，你可以使用laravel验证:

$validated = $request->validate([
    'title' => 'required|unique:usersTable,emailAddress|max:255',
]);

但是你也可以用这些方法:

1:

if (User::where('email',  $request->email)->exists())
{
  // object exists
} else {
  // object not found
}

2:

$user = User::where('email',  $request->email)->first();

if ($user)
{
  // object exists
} else {
  // object not found
}

3:

$user = User::where('email',  $request->email)->first();

if ($user->isNotEmpty())
{
  // object exists
} else {
  // object not found
}

4:

$user = User::where('email',  $request->email)->firstOrCreate([
      'email' => 'email'
],$request->all());

简单地用这个来判断真假 $ user =用户::(“电子邮件”、“=”,输入::('邮件'))- >存在(); 如果你想要$user with result，你可以使用这个， $ user =用户::(“电子邮件”、“=”,输入::('邮件'))- > ();

像这样检查结果，

if(count($user)>0){}

你也可以用这个， $ user =用户::(“电子邮件”、“=”,输入::('邮件')); 如果存在($ user - > ()) { $user = $user->get(); ｝

这取决于您是想在之后使用用户，还是只检查是否存在一个用户。

如果用户对象存在，你想使用它:

$user = User::where('email', '=', Input::get('email'))->first();
if ($user === null) {
   // user doesn't exist
}

如果你只是想检查一下

if (User::where('email', '=', Input::get('email'))->count() > 0) {
   // user found
}

或者更好

if (User::where('email', '=', Input::get('email'))->exists()) {
   // user found
}

Laravel 6或顶部:写表名，然后给出where子句条件，例如where('id'， $request->id)

 public function store(Request $request)
    {

        $target = DB:: table('categories')
                ->where('title', $request->name)
                ->get()->first();
        if ($target === null) { // do what ever you need to do
            $cat = new Category();
            $cat->title = $request->input('name');
            $cat->parent_id = $request->input('parent_id');
            $cat->user_id=auth()->user()->id;
            $cat->save();
            return redirect(route('cats.app'))->with('success', 'App created successfully.');

        }else{ // match found 
            return redirect(route('cats.app'))->with('error', 'App already exists.');
        }

    }

if($user->isEmpty()){
    // has no records
}

Eloquent使用集合。 参见以下链接:https://laravel.com/docs/5.4/eloquent-collections