php统计条数运行时报错

代码

//统计评论条数
$sql=mysqli_query($link,"select count(com_id) as c from comments com_id='$id'");
$data=mysqli_fetch_assoc($sql);
$count=$data['c'];
echo $count;
echo $id;//这个是商品id出得来

然后报错
Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result...

谢谢！

打印一下是否数据库连接错误或者sql语句错误

$sql=mysqli_query($link,"select count(com_id) as c from comments com_id='$id'");
if(!$sql) {
    printf("Error: %s\n", mysqli_error($link));
    exit();
}
$data=mysqli_fetch_assoc($sql);
$count=$data['c'];
echo $count;
echo $id;