As a developer, I expect that my HQL or XWQL queries will work the same whatever the database is used underneath. But it does not.
In MySQL, this query works, meanwhile it does not with HSQLDB:
The problem is that, with HSQL, we cannot do GROUP BY with only 1 column:
When you use GROUP BY, all your selected columns must be in the GROUP BY list, except any column that is an aggregate.