Correct syntax query insert in to table(MySql)

Question

I know that this is mysqli, but im wondring how to get this query correct. I want to insert username, besvarelse and modulid into table. but if besvarelse is empty or null i want to write back message that its allready delivered.

$sql  = "INSERT INTO oppgave(username, besvarelse, modulid) 
         select '$username', '$besvarelse', '$modulid'
         from dual
         where not exists (select modulid from oppgave where modulid='$modulid')";

$result=mysql_query($sql, $tilkobling);
if(mysql_affected_rows()==0){
      echo "<h1>Allready delivered</h1>";
}else{
      echo "<h1>Delivery ok</h1>";
}
?>

Answer 1

//«if besvarelse is empty or null i want to write back message that its allready delivered.»
//To check besvarelse we shold not use SQL, it is much better to check it with PHP

if ($besvarelse=='' or is_null($besvarelse)) {
    $resp = "<h1>Allready delivered</h1>";    
} else {
    $sql  = ......;
    $result=mysql_query($sql, $tilkobling);
    if(mysql_affected_rows()==0){
        $resp="<h1>Allready delivered</h1>";
    }else{
        $resp="<h1>Delivery ok</h1>";
}
echo $resp;