SQL & PL/SQL

I want to create a query that is a union such that the 2nd resultset is based on the 1st resultset. I have a table that has parent/child records in the same table.

Table: EVENTS
EVENT_ID
PARENT_EVENT_ID
CREATED_DATE
(other columns)

if PARENT_EVENT_ID is null then it is a parent record, else it is a child record. I want to select all parent records then union them with all the associated child records...something like this:

select * from EVENTS where CREATED_DATE < sysdate - 90 and PARENT_EVENT_ID is null -- All parents
union
select * from EVENTS where PARENT_EVENT_ID in (select EVENT_ID from EVENTS where CREATED_DATE < sysdate - 90 and PARENT_EVENT_ID is null) -- include any children of parents selected from above

This works but it's kind of ugly, I want to avoid using the sub-select in the 2nd because it is a repeat of the 1st statement, is there a way to alias the first statement and just refer to it in the 2nd query?