SQL (the language that is, as defined by ISO) has COLLECT and UNNEST operators which come reasonably close (not 100% of course) to "Date's concept of relation-v

SQL (the language that is, as defined by ISO) has COLLECT and UNNEST operators which come reasonably close (not 100% of course) to "Date's concept of relation-valued attributes". Moreover, I'm told those two operators are supported by at least one of the three big dogs.