Inconsistent behaviour of GROUP BY between MySQL and HSQLDB

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:

SELECT prop1, prop2 FROM Table GROUP BY prop1

The problem is that, with HSQL, we cannot do GROUP BY with only 1 column:
http://stackoverflow.com/questions/11627814/hsql-view-error-expression-not-in-aggregate-or-group-by-columns#11629835

When you use GROUP BY, all your selected columns must be in the GROUP BY list, except any column that is an aggregate.

Use case: XWIKI-10189.