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mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource
$sql = "SELECT * FROM $tbl_name WHERE....
 $result=mysql_query($sql);
 $row = mysql_fetch_assoc($result);

print mysql_error();

$sql = "SELECT * FROM $tbl_name WHERE....";

if ($result = mysql_query($sql)) {
    $row = mysql_fetch_assoc($result);
}
else print mysql_error();

$sql = "SELECT * FROM $tbl_name WHERE....";
$res = mysql_query($sql) or trigger_error(mysql_error()." in ".$sql);
$row = mysql_fetch_assoc($result);

or die(mysql_error());