Here's my original question: merging two data sets

Unfortunately I omitted some intircacies, that I'd like to elaborate here.

So I have two tables events_source_1 and events_source_2 tables. I have to produce the data set from those tables into resultant dataset (that I'd be able to insert into third table, but that's irrelevant).

events_source_1 contain historic event data and I have to do get the most recent event (for such I'm doing the following:

select event_type,b,c,max(event_date),null next_event_date
from events_source_1
group by event_type,b,c,event_date,null

events_source_2 contain the future event data and I have to do the following:

select event_type,b,c,null event_date, next_event_date
from events_source_2
where b>sysdate;

How to put outer join statement to fill the void (i.e. when same event_type,b,c found from event_source_2 then next_event_date will be filled with the first date found

GREATLY APPRECIATE FOR YOUR HELP IN ADVANCE.