Insert into table if two keys do not exist - mysql

Question

I have a table that maps itemId(item.id) and tagId(tag.id).
I'm writing a query that needs to insert a new row, but only if BOTH itemId and tagId do not already have an entry. What's the best way to do this?
Here is my current query that doesn't already check for an existing match:
// create tag-item maps
$sql = 'INSERT INTO item_tag
SELECT :itemId AS iid, tag.id AS tid
FROM tag
WHERE tag.name=:name';
try{
$stmt = $db->dbh->prepare($sql);
for($i = 0; $i < $count; $i++) {
$data = array(':itemId' => $itemId, ':name' => $tags[$i]);
$result = $stmt->execute($data);
if($result !== false) {
// Do nothing
}else {
return false;
}
}
return true;
}catch(PDOException $e) {
logit($e->getMessage());
return false;
}

Answer 1

A UNIQUE constraint would be the ideal solution, and according to the docs the MyISAM engine supports them.
ALTER TABLE item_tag ADD CONSTRAINT item_tag_iid_tid_uq UNIQUE (iid, tid);
Then if your INSERT tries to add a duplicate (iid, tid) pair it will throw and you can handle it in your catch code.
If you don't want to do the UNIQUE constraint, you could instead add an "existence" check to your INSERT query:
INSERT INTO item_tag
SELECT :itemId AS iid, tag.id AS tid
FROM tag
WHERE tag.name=:name' AND NOT EXISTS (
SELECT * FROM item_tag WHERE iid=:itemId AND tid=tag.id)
I tested this using my own differently-named tables, so hopefully I "translated" it correctly to your table/column names.