Magento: Very Useful Collection Functions

/**

* Get Zend_Db_Select instance

*/

$collection->getSelect();

/**

* Get collection size

*/

$collection->getSelect()->getSize();

/**

* Get sql select string or object

*/

$collection->getSelect()->getSelectSql();

/**

* Add select order

*/

$collection->getSelect()->setOrder($field, $direction);

/**

* Add field filter to collection

*

* If $attribute is an array will add OR condition with following format:

* array(

*     array('attribute'=>'firstname', 'like'=>'test%'),

*     array('attribute'=>'lastname', 'like'=>'test%'),

* )

*/

$collection->getSelect()->setOrder($field, $condition);

/**

* Set select distinct

*/

$collection->getSelect()->distinct($flag);

/**

* Get all data array for collection

*/

$collection->getSelect()->getData();

/**

* Reset loaded for collection data array

*/

$collection->getSelect()->resetData();

/**

* Print and/or log query

*/

$collection->getSelect()->printLogQuery(true, true);

More functions below:-
Varien_Data_Collection_Db class extends Varien_Data_Collection class. Here are some more functions present in Varien_Data_Collection class:-

/**

* Get current collection page

*/

$collection->getSelect()->getCurPage();

/**

* Retrieve collection last page number

*/

$collection->getSelect()->getLastPageNumber();

/**

* Retrieve collection page size

*/

$collection->getSelect()->getPageSize();

/**

* Retrieve collection all items count

*/

$collection->getSelect()->getSize();

/**

* Retrieve collection first item

*/

$collection->getSelect()->getFirstItem();

/**

* Retrieve collection last item

*/

$collection->getSelect()->getLastItem();

/**

* Retrieve collection items

*/

$collection->getSelect()->getItems();

/**

* Clear collection

*/

$collection->getSelect()->clear();

You can also use the select functions as present in Zend_Db_Select class. This class file is present in lib/Zend/Db/Select.php