how to join a table on a subquery that uses order by and limit - sql

Question

For each row from table tClass matching a given where clause,
join on the first row in tEv, sorted by time, where tEv.class_id = tClass.class_id
The following code throws the error
ORA-01799: a column may not be outer-joined to a subquery
select
c.class_id,
c.class_name,
e.start_time,
e.ev_id
from
tClass c
left join tEv e on (
e.ev_id = (
select
ss1.ev_id
from (
select
ed.ev_id
from
tEvDisp ed,
tEv e
where
ed.class_id = c.class_id
and ed.viewable = 'Y'
and ed.display_until > localtimestamp
and e.ev_id = ed.ev_id
order by
e.start_time
) ss1
where
rownum = 1
)
)
where
c.is_matching = 'Y';
How can this be rewritten to do what is described?
The above is for oracle, but needs to work in sqlite (substituting where necessary)

Answer 1

No idea about SQLite - that would need to be a separate question if this doesn't work - but for Oracle you could do something like this:
select c.class_id,
c.class_name,
e.start_time,
e.ev_id
from tClass c
left join (
select class_id, ev_id, start_time
from (
select ed.class_id,
ed.ev_id,
e.start_time,
row_number() over (partition by ed.class_id order by e.start_time) as rn
from tEvDisp ed
join tEv e on e.ev_id = ed.ev_id
where ed.viewable = 'Y'
and ed.display_until > localtimestamp
)
where rn = 1
) e on e.class_id = c.class_id
where c.is_matching = 'Y';
This uses a subquery which finds the most tEv data, using an analytic row_number() to identify the latest data for each class_id, which is restricted by the rn = 1 filter.
That subquery, consisting of at most one row per class_id, is then used the left outer join against tClass.

Answer 2

This sort of construct should get you what you need. You can fix the details.
select c.classid
, c.classname
, temp.maxstarttime
from tClass c left join (
select c.classid id
max(e.start_time) maxstarttime
from tClass join tEv on tEv.classId = tClass.ClassId
where whatever
group by c.classid) temp on c.classid = temp.id