How to use mysql select query in magento?

$cat_count = Mage::registry('current_category');
$query = 'SELECT COUNT(catalog_category_product_index.category_id) AS count FROM catalog_category_product_index WHERE catalog_category_product_index.category_id = '.print_r($cat_count['entity_id']).' GROUP BY catalog_category_product_index.category_id';
$query = Mage::getSingleton('core/resource')->getConnection('core_read')->query($query);
//and now you can build up your array or do anything you want (:
while($row=$query->fetch()) {
    $catCount[$row['category_id']]=$row['count'];
echo $row['count'];
}