yii\db\QueryBuilder::buildGroupBy PHP Method

    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);
    }