<?php

$model = new blogTagPluginModel();
$ids = array_keys($model->query("\n            SELECT t.id, COUNT(pt.tag_id) cnt FROM `blog_tag` t \n            LEFT JOIN `blog_post_tag` pt ON t.id = pt.tag_id\n            GROUP BY t.id\n            HAVING cnt = 0\n        ")->fetchAll('id'));
if ($ids) {
    $model->deleteById($ids);
}