QUERY:

SELECT orders_products.order_id FROM orders_products INNER JOIN orders ON orders.id = orders_products.order_id WHERE orders_products.productcode = 26232 AND orders.paid = 1;

EXPLAIN:

+----+-------------+-----------------+------+--------------------+--------+---------+------------------------+-------+-------------+
| id | select_type | table           | type | possible_keys      | key    | key_len | ref                    | rows  | Extra       |
+----+-------------+-----------------+------+--------------------+--------+---------+------------------------+-------+-------------+
|  1 | SIMPLE      | orders          | ALL  | PRIMARY            | NULL   | NULL    | NULL                   | 24983 | Using where |
|  1 | SIMPLE      | orders_products | ref  | Search,productcode | Search | 3       | mydatabase.orders.id   |     1 | Using where |
+----+-------------+-----------------+------+--------------------+--------+---------+------------------------+-------+-------------+

SHOW CREATE TABLE orders:

CREATE TABLE `orders` (
  `id` mediumint(7) unsigned NOT NULL AUTO_INCREMENT,
  `status` tinyint(1) unsigned NOT NULL DEFAULT '0',
  `client_name` varchar(50) NOT NULL DEFAULT '',
  `client_vatnumber` varchar(16) DEFAULT NULL,
  `client_address1` varchar(100) NOT NULL DEFAULT '',
  `client_address2` varchar(50) NOT NULL DEFAULT '',
  `client_email` varchar(80) NOT NULL DEFAULT '',
  `client_phone` varchar(30) NOT NULL DEFAULT '',
  `client_birthday` date DEFAULT NULL,
  `referencenumber` varchar(20) NOT NULL DEFAULT '',
  `info` text,
  `totalprice` decimal(9,2) NOT NULL DEFAULT '0.00',
  `added` datetime NOT NULL,
  `paid` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `Search` (`status`,`client_email`,`referencenumber`,`totalprice`),
  KEY `client_name` (`client_name`)
) ENGINE=MyISAM AUTO_INCREMENT=47974 DEFAULT CHARSET=latin1

SHOW CREATE TABLE orders_products:

CREATE TABLE `orders_products` (
  `ProductOrderID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `order_id` mediumint(7) unsigned NOT NULL DEFAULT '0',
  `count` tinyint(1) NOT NULL DEFAULT '0',
  `productcode` varchar(10) NOT NULL DEFAULT '',
  `prodtype` tinyint(3) unsigned NOT NULL DEFAULT '0',
  `name` varchar(80) NOT NULL DEFAULT '',
  `price` decimal(9,2) NOT NULL DEFAULT '0.00',
  `ProductReleaseDate` date DEFAULT '0000-00-00',
  PRIMARY KEY (`ProductOrderID`),
  KEY `Search` (`order_id`,`price`,`name`),
  KEY `productcode` (`productcode`),
) ENGINE=MyISAM AUTO_INCREMENT=61460 DEFAULT CHARSET=latin1