Are you sure you are looking for SQL and not AR filtering criteria?
Here is an example of SQL in AR (to get the maximum value in a column using a WHERE condition):
In controller:
$max = $model->getMax($filter);
In model:
public $maxColumn = 0;
public function getMax($filter = null)
{
/* Get max value from column1 WHERE column2 = $filter */
If($filter == null){return false;}
$criteria = new CDbCriteria;
$criteria->select = 'MAX(t.column1) as maxColumn';
$criteria->condition = 't.column2 LIKE :parm';
$criteria->params = array(':parm'=>$filter);
$tempmodel = $this->find($criteria);
$max = $tempmodel['maxColumn'];
return $max;
}