public function buildGroupBy($columns)
{
if (empty($columns)) {
return '';
}
foreach ($columns as $i => $column) {
if ($column instanceof Expression) {
$columns[$i] = $column->expression;
} elseif (strpos($column, '(') === false) {
$columns[$i] = $this->db->quoteColumnName($column);
}
}
return 'GROUP BY ' . implode(', ', $columns);
}