For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
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FROM `fab_rnk_u21`
LEFT JOIN `fab_months` AS `fab_months` ON `fab_months`.`id` = `fab_rnk_u21`.`mnth`
WHERE ( LOWER(`fab_rnk_u21`.`name`) regexp LOWER('rembert') )
ORDER BY `fab_rnk_u21`.`yr` DESC, `fab_months`.`month` DESC LIMIT 50
I think so, yes. In Prefilters there's a xy(raw) selection for dbjoin elements but not in the 'Order by'.
You could also add one more dbjoin with id/id (watching the other one or filled via php plugin) and use this for ordering.