查询错误 - 警告：mysqli_fetch_assoc（）[重复]

<?php
$con=mysqli_connect("localhost","xxx","xxx","xx");
// Check connection
if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

$sql = mysqli_query($con,"SELECT username, COUNT(username) AS total FROM xxxxx GROUP BY username");

$array = array();
while($fetch = mysqli_fetch_assoc($sql))
{
    $array[$fetch['username']] = $fetch['total'];
}

echo '<pre>'; print_r($array);

mysqli_close($con);
?>

Below is the error that i am receiving

Error::: Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, boolean given in /home/xxxxx.php on line 20

Thanks for help

</div>

try this as its not mysql but mysqli

while($fetch = sql->fetch_assoc())
{
......