[SQL, PHP]: Fetch rows from two column ordered lists by specifying IDs

NotificationRepository.php

<?php

    /**
     * Returns customer notifications
     *
     * @param int|null $limit
     * @param int|null $afterId
     * @param int|null $beforeId
     *
     * [0]*************************************[totalRows]
     * [0]*****[$beforeId]---------------------[totalRows]
     * [0]-----[$afterId]**********************[totalRows]
     * [0]-----[$afterId]******[$beforeId]-----[totalRows]
     *
     * $limit can be applied to all cases.
     * For example, 10 stars (rows) starting from the left will be returned.
     *
     * @return array
     */
    public function getNotifications($limit = null, $afterId = null, $beforeId = null)
    {
        $sql = new DbQuery();
        $sql->select('id_notification, icon, title, url, message, seen, date_add')
            ->from('notification')
            ->where('id_customer = '.(int)$this->id_customer)
            ->orderBy('date_add DESC')
            ->orderBy('id_notification DESC');

        if (null !== $limit) {
            $sql->limit((int)$limit);
        }

        if (null !== $afterId) {
            $a = $this->getNotification($afterId, array('date_add'));
            $sql->where(
                '     (date_add = \''.pSQL($a['date_add']).'\' AND id_notification < '.(int)$afterId.')'
                .' OR (date_add < \''.pSQL($a['date_add']).'\')'
            );
        }

        if (null !== $beforeId) {
            $b = $this->getNotification($beforeId, array('date_add'));
            $sql->where(
                '     (date_add = \''.pSQL($b['date_add']).'\' AND id_notification > '.(int)$beforeId.')'
                .' OR (date_add > \''.pSQL($b['date_add']).'\')'
            );

            /**
             * Nonsensical query?
             * [0]-----[beforeId]-----[afterId]-----[totalRows]
             */
            if (!empty($a) && strtotime($b['date_add']) > strtotime($a['date_add'])) {
                return array();
            }
        }

        return (array)$this->db->executeS($sql);
    }