Delete table entry if not in current input

I have a form where users can enter tags like tag1,tag2,tag3. I explode that information and insert them into a table:tags one by one. Each tag in the table has a unique id (for referencing), a vid_id (for matching to a video), and a name (ex. tag1).

I want the behavior that if someone has those 3 tags in the table and then re-submits the form with tag开发者_开发知识库1,tag2 for the same vid_id that tag3 will be removed from the table. I was just thinking of a way to do this most efficiently and I was thinking of running a select statement each iteration or just deleting all the tags for that vid_id and reinserting tag1,tag2 but that seems inefficient. Anyone got any good ideas?

$variable=explode(',',$_POST['tags']);
foreach($variable as $tag) {
    $id=md5(uniqid());
    if ($tag!=''){
        $sql="INSERT into tags (id,vid_id,name) VALUES (?,?,?)";
        $stmt16 = $conn->prepare($sql);
        $result=$stmt16->execute(array($id,$vid_id,$tag));
    }
}

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I don't fully understand the question. Pieces that might help you: WHERE NOT IN SQL syntax:

DELETE FROM tags     
WHERE tags.name NOT IN ('tag1', 'tag2', 'tag3') 

and REPLACE statement: http://dev.mysql.com/doc/refman/5.0/en/replace.html