protected function generateSelectSQL()
        {
            $sql = ($this->query->isDistinct()) ? 'SELECT DISTINCT ' : 'SELECT ';

            if ($this->query->isCustomSelection()) {
                if ($this->query->isDistinct() && Core::getDriver() == Core::DRIVER_POSTGRES) {
                    foreach ($this->query->getSortOrders() as $sort_order) {
                        $this->query->addSelectionColumn($sort_order->getColumn());
                    }
                }

                $sql_parts = [];
                foreach ($this->query->getSelectionColumns() as $column => $selection) {
                    $alias = ($selection->getAlias()) ?? $this->query->getSelectionAlias($column);
                    $sub_sql = $selection->getVariableString();
                    if ($selection->isSpecial()) {
                        $sub_sql .= mb_strtoupper($selection->getSpecial()) . '(' . Query::quoteIdentifier($selection->getColumn()) . ')';
                        if ($selection->hasAdditional())
                            $sub_sql .= ' ' . $selection->getAdditional() . ' ';
                        if (mb_strpos($selection->getSpecial(), '(') !== false)
                            $sub_sql .= ')';
                    } else {
                        $sub_sql .= Query::quoteIdentifier($selection->getColumn());
                    }
                    $sub_sql .= ' AS ' . Query::quoteIdentifier($alias);
                    $sql_parts[] = $sub_sql;
                }
                $sql .= implode(', ', $sql_parts);
            } else {
                $this->addAllSelectColumns();
                $sql .= $this->generateSelectAllSQL();
            }

            return $sql;
        }