如果查询结果为空，则回显错误消息

How can I add another error message if the query is empty? I tried with mysqli_num_rows without success. :/

$que = new mysqli($host, $db_user, $db_password, $db_name);
$tbl = $que->query
("
 SELECT iditem, nazwa
 FROM  `idlist2` 
 WHERE nazwa LIKE  '%$name%'
");

while($value=$tbl->fetch_assoc())
{
    if(strlen($name) == 0 || strlen($name) == NULL)
    {
        echo '<font color="green">!</font>';
        break;
    }
    echo $value['iditem'] . $value['nazwa'] . "<br>";
}

Here's one way to do it:

while($value=$tbl->fetch_assoc())
{
    $notempty = true;                              // set a variable if you fetch anything
    if(strlen($name) == 0 || strlen($name) == NULL)
    {
        echo '<font color="green">!</font>';
         break;
    }
    echo $value['iditem'] . $value['nazwa'] . "<br>";
}
if (!isset($notempty)) {
    echo 'other error message';                   // show the other message if it isn't set
}

You are actually doing this the wrong way. mysqli_num_rows is used when you are querying in a procedural manner.

In your code sample, you created a new object by calling new mysqli(). To get the number of rows fetched when using OOP construct, you should get the value of num_rows property. And this is how to go about that:

if ($tbl->num_rows > 0) {
    while($value=$tbl->fetch_assoc())
    {
        if(strlen($name) == 0 || strlen($name) == NULL)
        {
            echo '<font color="green">!</font>';
            break;
        }
        echo $value['iditem'] . $value['nazwa'] . "<br>";
    }
} else {
        echo "No result found";
}
$que->close();