如何解决错误：查询中的查询为空？

我正在从php更新数据库中的数据，并且出现Up Up Error：Query是空的查询。我非常需要帮助。提前致谢

<?php
$dbhost = 'localhost';
$dbuser = 'root';
$myDBname = 'jsv';
$conn = mysql_connect("localhost", "root");
mysql_select_db('jsv');
if(isset($_POST['update']))
{
$nam = $_POST['namnid'];
$mandag = $_POST['mandagid'];
$tisdag = $_POST['tisdagid'];
$torsdag = $_POST['torsdagid'];
$fredag = $_POST['fredagid'];

$sql = mysql_query("INSERT INTO jsv(CA_ID,Name,Address,Amount)  VALUES ('$mandag', '$tisdag', '$torsdag', '$fredag') WHERE Setup_Box_No = '$nam'", $conn);
if(!$sql )
{
  die('Could not update data: ' . mysql_error());
}
echo "Updated data successfully\n";

}

mysql_close($conn);
?>