Hibernate User Guide relevant section: https://docs.jboss.org/hibernate/orm/6.6/userguide/html_single/Hibernate_User_Guide.html#hql-aggregation

Support HQL grouping. This involves support for

• The GROUP BY clause
• The HAVING clause
• Aggregate functions (e.g. COUNT, MIN, MAX, SUM, AVG)

These can be translated into MQL using $group (for GROUP BY), $match (for HAVING), and group accumulators like $count, $first, $last, $sum, $avg.  $first and $last will require a $sort stage as well, which is implicit in HQL but must be explicit in MQL.