You can access the database via WordPress. You can use any raw SQL too. That is quite easy. You only need to call a global object;
global $wpdb;
For the raw SQL, you can use query method:
$wpdb->query(
"
UPDATE $wpdb->posts
SET post_parent = 10
WHERE ID = 15

"
);

For get the record set,
$drafts = $wpdb->get_results(
"
SELECT *
FROM $wpdb->posts
WHERE post_status = 'draft'
AND post_author =1
"
);

There are insert, update and delete method too.
Please read WordPress development documentation for the further information