php - Uncaught Error: Call to a member function fetch_assoc() on boolean

Im trying to learn php
Im using php 7.0.1
Im using php mysqli
I saw a lot of problem like this but their solutions doesnt help me i think its because im using latest version and others uses previous version
heres my php script:

session_start();
require_once '../../../db_connect.php';

$result = $db->query("select * from student where id = 1");


$personalinformation = $result->fetch_assoc();
echo $personalinformation['username'];
?>

heres my student table:

id | username | password
1  | user1    | pass1

2  | user2    | pass2
3  | user3    | pass3