为什么我的这样？SQLSTATE[HY000] [1049] Unknown database 'root' (SQL: select * from student)

<?php

    namespace App\Http\Controllers;

use Illuminate\Support\Facades\DB;

class StudentController extends Controller {

        public function test1 () {

            $students = DB::select('select * from student');

            var_dump($students);

    }