mysql don't post dupes to table - php

Question

So I have a database with a table of movies with their respective directors.
I have a form where one can add a director to a movie, but I want to not add duplicates (i.e. someone adds a director to a movie that is already in the movie director table.
I have the following, but for the life of me, I cannot figure out why this still adds the duplicate:
if (isset($_POST["submit"])) {
$movie_id = $_POST[movie];
$director_id = $_POST[director];
echo $movie_id;
echo '<br/>';
echo $director_id;
echo '<br/>';
$add_sql = "INSERT IGNORE INTO MovieDirector (mid, did)
VALUES ($movie_id, $director_id)";
if (mysql_query($add_sql, $db_connection)){
echo "added director to movie ";
}
else {
echo "Failed ";
}

Answer 1

I think you are confused about what the IGNORE modifier keyword does for INSERT statements in MySQL. The role of IGNORE is not to prevent duplicate records from happening, but to make errors silent.
So, there are multiple ways you could go about this:
you could modify your schema to not allow duplicate records and either continue to use the IGNORE keyword, or better yet, handle theerrors
ALTER TABLE MovieDirector ADD UNIQUE INDEX(mid, did);
you could also modify your query to not insert a record if one already exists
IF NOT EXISTS(SELECT * FROM MovieDirector WHERE mid = $movieId AND did = $directorId)
INSERT INTO MovieDirector (mid, did) VALUES ($movieId, $directorId)
You probably want both of the above (probably minus the IGNORE keyword.. that's just going to come and bite you later)
This answer would be incomplete if I did not stress that you should really be using parametrized queries instead of adding the variables directly into the query string!