In Workplace we have a unittest that tests re-enrolment and recovering course grades. This test started failing on MariaDB (on ci.moodle.org and in moodle-docker).

There are no tests for the function grade_recover_history_grades() in Moodle LMS.

To be honest I don't understand what exactly is happening - why did it suddenly start falling now and why the current SQL query does not work on mariadb (by "does not work" I mean that this query was returning empty array in mariadb)

But what I can do is to add the same unittest to Moodle LMS and change the SQL query so it works on all dbs.

To reproduce:

from the attached solution take only the unittest and run it on different DB engines on ci.moodle.org or in moodle-docker - it will work on postgres/mysql/oracle/mssql but will fail on mariadb.