SQL (snowflake) - how can I return 1 row from a join or use MAX in a second join from result of first - sql

Question

I have a large query that I have pasted parts of below.
I am wanting to use the result of the first join in my second join.
What I am trying to do get the last session that has a lead_conversion then I am getting all sessions in between then and the current row
This is the part I am struggling with
left join (
select ss.id, ss.session_start, ss.lead_id
from sessions ss
inner join lead_conversions inner_lc on inner_lc.session_id = ss.id
) prev_lc
on prev_lc.lead_id = lc.lead_id
and prev_lc.session_start::TIMESTAMP < s.session_start::TIMESTAMP
left join cte_sessions reset_prev_sess
on reset_prev_sess.lead_id = lc.lead_id
and reset_prev_sess.session_start::TIMESTAMP <= s.session_start::TIMESTAMP
and (
prev_lc.session_start::TIMESTAMP IS NULL
OR
reset_prev_sess.session_start::TIMESTAMP > prev_lc.session_start::TIMESTAMP
)
my issue is I cant just fetch the last prev_lc and I cant seem to use max(prev_lc.session_start)
I have tried grouping in first select and using max but this does not work as I believe this is ran before the on
left join (
select max(ss.session_start) as session_start, max(ss.lead_id) as lead_id
from sessions ss
inner join lead_conversions inner_lc on inner_lc.session_id = ss.id
group by inner_lc.id
) prev_lc on prev_lc.lead_id = lc.lead_id
I have also tried using max in the second join but this give the error
SQL compilation error: Invalid aggregate function in ON clause [MAX(CAST(PREV_LC.SESSION_START AS TIMESTAMP_NTZ(9)))]
left join cte_sessions reset_prev_sess
on reset_prev_sess.lead_id = lc.lead_id
and reset_prev_sess.session_start::TIMESTAMP <= s.session_start::TIMESTAMP
and (
prev_lc.session_start::TIMESTAMP IS NULL
OR
reset_prev_sess.session_start::TIMESTAMP > max(prev_lc.session_start::TIMESTAMP)
)
any help with this would be very appreciated
Thank you

Answer 1

if I understand correctly you are looking for to join with the last session start,so what you can do is to order by startsession in your subquery and limit to 1 record:
left join (
select ss.id, ss.session_start, ss.lead_id
from sessions ss
inner join lead_conversions inner_lc on inner_lc.session_id = ss.id
order by ss.session_start desc
limit 1
) prev_lc
the rest of query stays untouched.

Answer 2

So I have found a solution for this if any one comes across this. I ended up just rethinking how I go about it.
I ended up adding a row number for each conversion
with cte_sessions as (
select
s.id
,s.lead_id
,s.session_start::TIMESTAMP as session_start
,CASE WHEN MAX(lc.id) IS NOT NULL
then ROW_NUMBER() over (partition by s.lead_id, (CASE WHEN
MAX(lc.id) IS NOT NULL then 1 else 0 end)
order by s.session_start
)
END as conversion_row
from sessions s
left join lead_conversions lc on lc.session_id = s.id
group by s.id, s.session_start, s.lead_id, s.project_id, s.crawler_id
order by s.session_start
)
The I just did this in the join
left join cte_sessions prev_lc on prev_lc.lead_id = lc.lead_id and prev_lc.conversion_row = s.conversion_row - 1