Postgres group by columns and within group select other columns by max aggregate - sql

Question

This is probably a standard problem, and I've keyed off some other greatest-n-per-group answers, but so far been unable to resolve my current problem.
A B C
+----+-------+ +----+------+ +----+------+-------+
| id | start | | id | a_id | | id | b_id | name |
+----+-------+ +----+------+ +----+------+-------+
| 1 | 1 | | 1 | 1 | | 1 | 1 | aname |
| 2 | 2 | | 2 | 1 | | 2 | 2 | aname |
+----+-------+ | 3 | 2 | | 3 | 3 | aname |
+----+------+ | 4 | 3 | bname |
+----+------+-------+
In English what I'd like to accomplish is:
For each c.name, select its newest entry based on the start time in a.start
The SQL I've tried is the following:
SELECT a.id, a.start, c.id, c.name
FROM a
INNER JOIN (
SELECT id, MAX(start) as start
FROM a
GROUP BY id
) a2 ON a.id = a2.id AND a.start = a2.start
JOIN b
ON a.id = b.a_id
JOIN c
on b.id = c.b_id
GROUP BY c.name;
It fails with errors such as:
ERROR: column "a.id" must appear in the GROUP BY clause or be used in an aggregate function Position: 8
To be useful I really need the ids from the query, but cannot group on them since they are unique. Here is an example of output I'd love for the first case above:
+------+---------+------+--------+
| a.id | a.start | c.id | c.name |
+------+---------+------+--------+
| 2 | 2 | 3 | aname |
| 2 | 2 | 4 | bname |
+------+---------+------+--------+
Here is a Sqlfiddle
Edit - removed second case

Answer 1

Case 1
select distinct on (c.name)
a.id, a.start, c.id, c.name
from
a
inner join
b on a.id = b.a_id
inner join
c on b.id = c.b_id
order by c.name, a.start desc
;
id | start | id | name
----+-------+----+-------
2 | 2 | 3 | aname
2 | 2 | 4 | bname
Case 2
select distinct on (c.name)
a.id, a.start, c.id, c.name
from
a
inner join
b on a.id = b.a_id
inner join
c on b.id = c.b_id
where
b.a_id in (
select a_id
from b
group by a_id
having count(*) > 1
)
order by c.name, a.start desc
;
id | start | id | name
----+-------+----+-------
1 | 1 | 1 | aname