Mysql Query to Drupal SChema Query How to

I just want to escape-disabling the create-form to appear at the user when he've already submit contest at this content type and the specific taxonomy term.
e.g
I want to create a submit to contest content type.I want the query only from the database ( i dont care if i use the schema or not ) in order not having double posts at the same contest from the same user.
So if it was a simple mysql-query at a custom cms it could be like this

$query="select * from posts where taxonomy_term=$term and user=$current_user";
$result = mysql_query($query);
if(mysql_num_rows($result)>0){
echo "U've already post at this contest.Sorry next time";
}else{
//Continue with drupal code
}

THanx!