How would you write this query efficiently?

Question

I am having a problem doing a particular join in the most optimal fashion, and this kind of join I do often, so I'd like to learn the best way to do it so I can re-use the same logic every time and be comfortable that it is the most efficient means.
I am using Oracle 11g, and the situation goes like this -- well actually there is a variation of this particular situation, I'll explain the easier one first):
Table PARENT (
PARENT_ID NUMBER [PK],
(OTHER IRRELEVANT DATA COLUMNS)
Table CHILD (
CHILD_ID NUMBER [PK],
PARENT_ID NUMBER,
CHILD_VALUE VARCHAR2(50),
START_DATE DATE NULL(NO),
END_DATE DATE NULL(YES)
I want to query from PARENT, joining to CHILD when possible (e.g. an outer join) but I only want the MOST RECENT record. MOST RECENT means that it is the record with the latest START_DATE, which is a column that cannot be NULL.
I usually try a query where I make a inner SELECT in the FROM clauses, then do an outer join against it. But this seems ot be a terrible practice that slows the query down drastically, because looks like the DB analyzes that sub query for the entire DB before joining on my particular result set. Like the main query might filter it down very quickly to a small set of records, but that sub query still analyzes the entire table then joins to my subset. So something like this:
SELECT
p.(WHATEVER COLUMNS FROM PARENT),
x.CHILD_VALUE,
x.START_DATE,
x.END_DATE
FROM PARENT p,
(SELECT c1.PARENT_ID, c1.CHILD_VALUE, c1.START_DATE, c1.END_DATE
FROM CHILD c1
WHERE c1.START_DATE = (SELECT MAX(START_DATE) FROM CHILD)
) x
WHERE
p.(FILTERING ON PARENT)
AND x.PARENT_ID(+) = p.PARENT_ID
Then a variation of this is that the CHILD table has one other column:
CHILD_TYPE (NUMBER)
And I need to join on the CHILD getting the MOST RECENT record for each CHILD_TYPE... for this I do two inner queries usually, and again I know its not the right way to do it.

Answer 1

with most_recent_child as (
                           select  *
                             from  (
                                    select  c.*,
                                            row_number() over(partition by parent_id order by start_date desc) rn
                                            from  child c
                             where rn = 1
select  *
  from      parent p
        left join
            most_recent_child c
          on p.parent_id = c.parent_id
/SY.