How can I write this sql query following the Magento Way?

Question

How can I write this unsecure, ugly, query following the Magento way without using collection?

select (select sum(points) FROM ".$this->getTable('mymodule/registry')." where points < 0 and entity_id=".$customerId.") as points_spent,
(select sum(points) FROM ".$this->getTable('mymodule/registry')." where points > 0  and entity_id=".$customerId.") as points_received,
(select sum(points) FROM ".$this->getTable('mymodule/registry')."  where entity_id=".$customerId.") as points_current

EDIT

A possible solution.

In a resource model I can put something like this:

$adapter = $this->_getReadAdapter();

        $receivedPoints = $adapter->select()
            ->from(array('mymodule/registry'), array(new Zend_Db_Expr('sum(points)')))
            ->where('points > ?', 0)
            ->where('entity_id = ?', $customerId);

        $spentPoints = $adapter->select()
            ->from(array('mymodule/registry'), array(new Zend_Db_Expr('sum(points)')))
            ->where('points < ?', 0)
            ->where('entity_id = ?', $customerId);

        $query = $adapter->select()
            ->from(
                array('mymodule/registry'),
                array('current_points'  => new Zend_Db_Expr('sum(points)'),
                      'received_points' => new Zend_Db_Expr('(' . $receivedPoints . ')'),
                      'spent_points'    => new Zend_Db_Expr('(' . $spentPoints . ')')
                )
            )
            ->where('entity_id = ?', $customerId);
        return $adapter->fetchRow($query);

Answer 1

Try to add this function to your collection class. My logic is you have sum of points grouping by below of 0 and above of 0 and all of them.

class My_Module_Model_Mysql4_Registry_Collection extends Mage_Core_Model_Mysql4_Collection_Abstract
{
    public function _construct()
    {
        $this->_init('mymodule/registry');
    }

    public function getSumOfPoints($customerId)
    {
        $this->addFieldToFilter('customer_id', $customerId);

        $this->getSelect()
            ->reset('columns')
            ->columns(new Zend_Db_Expr("sign(points) as sign"))
            ->columns(new Zend_Db_Expr("sum(points) as summ"))
            ->group(new Zend_Db_Expr("sign(points)"));
        return $this;
    }
}

And you can get results by calling this function at anywhere:

$results = Mage::getResourceModel('mymodule/registry_collection')->getSumOfPoints($customerId);
foreach ($results as $result) {
    print_r($result->getData());
}

If you put all above correctly it will give result like these:

Array
(
    [sign] => -1 //summ of points less than 0
    [summ] => -166
)
Array
(
    [sign] => 1 //summ of points great than 0
    [summ] => 312
)

You can get summ of all points adding two result above. If you don't want to use collection class you can adopt this logic to your own code which is using resource model. Collection class generates sql query given below, try first this query, if it gives appropriate result you can use code given above:

SELECT
    sign(points)AS sign,
    sum(points)AS summ
FROM
    `mymodule_registry` AS `main_table` //put here real table name
WHERE
    (customer_id = '4') //put real customer id
GROUP BY
    sign(points)

Answer 2

At the end of the day, that part of Magento relies on Zend_DB, so what are you looking for is: Zend_Db_Select. lib/Varien (and in newer Magento version lib/Magento) extends this part.

The object used for selects is Varien_Db_Select which extends Zend_Db_Select

 $db = Mage::getSingleton('core/resource')->getConnection('core_read');//(Magento|Varien)_Db_Adapter_Pdo_Mysql
    $db->select();
    //does:
    public function select()
    {
        return new Varien_Db_Select($this);
    }