Warning: mysqli_fetch_array() expects parameter 1 to be mysqli_result, bool given in

<?php
//链接建立数据库服务器
$conn = mysqli_connect("localhost","root","989900","student");
$sql = "select * from sm201";
$res = mysqli_query($conn,$sql);
//循环取出每一条记录，判断该记录为合格良好优秀不合格
while($row=mysqli_fetch_array($res)){

    //echo $row['score']."<br>";
    switch($row['score']){
    case $row['score']>=90:
        $sql = "update sm201 set level='优秀' where id='".$row['id']."'";
        $res = mysqli_query($conn,$sql);
        break;
    case $row['score']>=80:
        $sql = "update sm201 set level='良好' where id='".$row['id']."'";
        $res = mysqli_query($conn,$sql);
        break;
    case $row['score']>=60:
        $sql = "update sm201 set level='及格' where id='".$row['id']."'";
        $res = mysqli_query($conn,$sql);
        break;
    default:
        $sql = "update sm201 set level='不合格' where id='".$row['id']."'";
        $res = mysqli_query($conn,$sql);
        break;
}
}

while后面引了 ，用下面的语句
echo mysqli_close($conn);
应该是你连接数据库错误，