MySQL：如果表中不存在变量，如何回显文本？

what my code does right now is echo the links from MySQL where movie_id is equal to the GET id. what I want to add is make it echo a message like "No links were fond" if there are no links where movie_id = id

<?php
// set the _GET id

if(isset($_GET["id"])){
    $id = preg_replace('#[^a-z0-9]#i', '', $_GET['id']);
} else {
    header("location: /");
    exit(); 
}

// display data

$query = "SELECT * FROM links WHERE movie_id='$id'";
$result = mysql_query($query) or die(mysql_error());
while($row = mysql_fetch_array($result)) {

echo "<a href=\"".$row['url']."\" target=\"_blank\">link</a>";
}
?>

Just add a check before the while loop.

if(mysql_num_rows($result) > 0)
{

    while($row = mysql_fetch_array($result)) {

        echo "<a href=\"".$row['url']."\" target=\"_blank\">link</a>";
    }
}else{
    echo "No records were found.";
}

If possible, use PDO extension for database queries. http://php.net/pdo

use empty()

    if(empty($result))
    {
    echo 'your link here';
    }
else
{
while($row = mysql_fetch_array($result)) {

echo "<a href=\"".$row['url']."\" target=\"_blank\">link</a>";
}
}

Or You can also use

if( mysql_num_rows($result) === 0 ){
    echo 'no link ';
}
else{
    while($row = mysql_fetch_array($result)) {
        echo "<a href=\"".$row['url']."\" target=\"_blank\">link</a>";
    }
}

Try to use PDO.MYSQL deprecated. Try like this :

 $result = mysql_query("SELECT * FROM links WHERE movie_id='$id'"); 
    if (mysql_num_rows($result)){
       while($row = mysql_fetch_array($result)) {

echo "<a href=\"".$row['url']."\" target=\"_blank\">link</a>";
}
}else{ echo "No Result"; }

mysql_num_rows() is used to get the number of rows.

if( mysql_num_rows($result) == 0 ){
    // Data not found
}
else{
    while($row = mysql_fetch_array($result)) {
        echo "<a href=\"".$row['url']."\" target=\"_blank\">link</a>";
    }
}

Use this simple code:

 if($result=="")
{
echo 'No link found';
}
else
{
--your link--
}

Consider using object-oriented mysqli and prepared statements:

#get the id from the paramater in the url
$id = $_GET['id'];

#initialize a mysqli object
$mysqli = new mysqli('host','user','pw','db');

#cache the query
$query = <<<Q
SELECT url FROM links WHERE movie_id=?
Q;

#initialize a prepared statement    
$stmt = $mysqli->stmt_init();

#returns true and evaluates to true if the query is doable
if($stmt->prepare($query)) {

   #binds your id the the ? in the query (this adds security to your script)
   $stmt->bind_param("i",$id);
   $stmt->execute();

   #gives each result in the column "url" a name, as the variable $url (which I assigned)
   $stmt->bind_result($url)

   # $stmt->fetch() returns false when no more results are found
   while ($stmt->fetch()) {
        echo $url;
   }
} else {

   #if no results were found through your query, echo "none found"
   echo 'None found';
}