Sitemap XML v3.9.6, running on zc155e, PHP 7.3 and MySQL 5.5.5-10.4.10-MariaDB (ONLY_FULL_GROUP_BY, STRICT_TRANS_TABLES, NO_ZERO_IN_DATE, NO_ZERO_DATE, ERROR_FOR_DIVISION_BY_ZERO, NO_AUTO_CREATE_USER, NO_ENGINE_SUBSTITUTION).

When products_reviews are part of the generated sitemap, I'm receiving a log similar to
Code:
[22-Jun-2021 18:52:36 Europe/Berlin] Request URI: /mysite/index.php?main_page=sitemapxml, IP address: ::1
#1  trigger_error() called at [C:\xampp\htdocs\mysite\includes\classes\db\mysql\query_factory.php:171]
#2  queryFactory->show_error() called at [C:\xampp\htdocs\mysite\includes\classes\db\mysql\query_factory.php:143]
#3  queryFactory->set_error() called at [C:\xampp\htdocs\mysite\includes\classes\db\mysql\query_factory.php:270]
#4  queryFactory->Execute() called at [C:\xampp\htdocs\mysite\includes\modules\pages\sitemapxml\sitemapxml_products_reviews.php:30]
#5  include(C:\xampp\htdocs\mysite\includes\modules\pages\sitemapxml\sitemapxml_products_reviews.php) called at [C:\xampp\htdocs\mysite\includes\templates\template_default\sitemapxml\tpl_main_page.php:21]
#6  require(C:\xampp\htdocs\mysite\includes\templates\template_default\sitemapxml\tpl_main_page.php) called at [C:\xampp\htdocs\mysite\index.php:97]
--> PHP Fatal error: 1055:'mydatabase.rd.languages_id' isn't in GROUP BY :: SELECT r.products_id, MAX(r.date_added) AS date_added, MAX(r.last_modified) AS last_modified, GREATEST(MAX(r.date_added), IFNULL(MAX(r.last_modified), '0001-01-01 00:00:00')) AS last_date, rd.languages_id
          FROM reviews r
            LEFT JOIN reviews_description rd ON (r.reviews_id = rd.reviews_id),
               products p
          WHERE p.products_id=r.products_id
            AND p.products_status=1
            AND r.status = 1
            AND rd.languages_id IN (1)
          GROUP BY r.products_id ORDER BY last_date DESC ==> (as called by) C:\xampp\htdocs\mysite\includes\modules\pages\sitemapxml\sitemapxml_products_reviews.php on line 30 <== in C:\xampp\htdocs\mysite\includes\classes\db\mysql\query_factory.php on line 171.
I corrected this by changing line 28 of /includes/modules/pages/sitemapxml/sitemapxml_products_reviews.php, adding the highlighted element:
Code:
  $sql = "SELECT r.products_id, MAX(r.date_added) AS date_added, MAX(r.last_modified) AS last_modified, GREATEST(MAX(r.date_added), IFNULL(MAX(r.last_modified), '0001-01-01 00:00:00')) AS last_date, rd.languages_id
          FROM " . TABLE_REVIEWS . " r
            LEFT JOIN " . TABLE_REVIEWS_DESCRIPTION . " rd ON (r.reviews_id = rd.reviews_id),
               " . TABLE_PRODUCTS . " p
          WHERE p.products_id=r.products_id
            AND p.products_status=1
            AND r.status = 1
            AND rd.languages_id IN (" . $sitemapXML->getLanguagesIDs() . ")
          GROUP BY r.products_id, rd.languages_id" .
          (SITEMAPXML_PRODUCTS_REVIEWS_ORDERBY != '' ? " ORDER BY " . SITEMAPXML_PRODUCTS_REVIEWS_ORDERBY : '');