protected function _createKeySql($fields) {
$return = array();
foreach ($fields as $field) {
if (is_array($field)) {
$return[] = 'substr(' . $field[0] . ', 1, ' . $field[1] . ')';
}
else {
$return[] = '"' . $field . '"';
}
}
return implode(', ', $return);
}