Does hsqldb provide a function similar to listagg?
group_concat is probably what you are looking for:
http://www.hsqldb.org/doc/2.0/guide/dataaccess-chapt.html#dac_aggregate_funcs
Quote from the manual:
GROUP_CONCAT is a specialised function derived from ARRAY_AGG. This function computes the array in the same way as ARRAY_AGG, removes all the NULL elements, then returns a string that is a concatenation of the elements of the array