Marco,
I just don't see the point in having a where clause in the main select
referencing the result of a subselect. And in my understanding, a select
should first limit its records by its where criteria and then get the field
values for those matching records. And I think subselects should be part of
getting those field values, so the value is not available at the time the
main where criteria is checked.

Furthermore, I cannot think of any situation where such a feature would be
useful (do you have any example where it is not equivalent to a join?).

Set