Join to table according to date - mysql

Question

I have two tables, one is a list of firms, the other is a list of jobs the firms have advertised with deadlines for application and start dates.
Some of the firms will have advertised no jobs, some will only have jobs that are past their deadline dates, some will only have live jobs and others will have past and live applications.
What I want to be able to show as a result of a query is a list of all the firms, with the nearest deadline they have, sorted by that deadline. So the result might look something like this (if today was 2015-01-01).
Sorry, I misstated that. What I want to be able to do is find the next future deadline, and if there is no future deadline then show the last past deadline. So in the first table below the BillyCo deadline has passed, but the next BuffyCo deadline is shown. In the BillyCo case there are earlier deadlines, but in the BuffyCo case there are both earlier and later deadlines.
id name title date
== ==== ===== ====
1 BobCo null null
2 BillCo Designer 2014-12-01
3 BuffyCo Admin 2015-01-31
So, BobCo has no jobs listed at all, BillCo has a deadline that has passed and BuffyCo has a deadline in the future.
The problematic part is that BillCo may have a set of jobs like this:
id title date desired hit
== ===== ==== ===========
1 Coder 2013-12-01
2 Manager 2014-06-30
3 Designer 2012-12-01 <--
And BuffyCo might have:
id title date desired hit
== ===== ==== ===========
1 Magician 2013-10-01
2 Teaboy 2014-05-19
3 Admin 2015-01-31 <--
4 Writer 2015-02-28
So, I can do something like:
select * from (
select * from firms
left join jobs on firms.id = jobs.firmid
order by date desc)
as t1 group by firmid;
Or, limit the jobs joined or returned by a date criterion, but I don't seem to be able to get the records I want returned. ie the above query would return:
id name title date
== ==== ===== ====
1 BobCo null null
2 BillCo Designer 2014-12-01
3 BuffyCo Writer 2015-02-28
For BuffyCo it's returning the Writer job rather than the Admin job.
Is it impossible with an SQL query? Any advice appreciated, thanks in advance.

Answer 1

I think this may be what you need, you need:
1) calculate the delta for all of your jobs between the date and the current date finding the min delta for each firm.
2) join firms to jobs only on where firm id's match and where the calculated min delta for the firm matches the delta for the row in jobs.
SELECT f.id, f.name, j.title,j.date
FROM firms f LEFT JOIN
(SELECT firmid,MIN(abs(datediff(date, curdate())))) AS delta
FROM jobs
GROUP BY firmid) d
ON f.id = d.firmid
LEFT JOIN jobs j ON f.id = j.id AND d.delta = abs(datediff(j.date, curdate())))) ;

Answer 2

You want to make an outer join with something akin to the group-wise maximum of (next upcoming, last expired):
SELECT * FROM firms LEFT JOIN (
-- fetch the "groupwise" record
jobs NATURAL JOIN (
-- using the relevant date for each firm
SELECT firmid, MAX(closest_date) date
FROM (
-- next upcoming deadline
SELECT firmid, MIN(date) closest_date
FROM jobs
WHERE date >= CURRENT_DATE
GROUP BY firmid
UNION ALL
-- most recent expired deadline
SELECT firmid, MAX(date)
FROM jobs
WHERE date < CURRENT_DATE
GROUP BY firmid
) closest_dates
GROUP BY firmid
) selected_dates
) ON jobs.firmid = firms.id
This will actually give you all jobs that have the best deadline date for each firm. If you want to restrict the results to an indeterminate record from each such group, you can add GROUP BY firms.id to the very end.

Answer 3

The revision to your question makes it rather trickier, but it can still be done. Try this:
select
closest_job.*, firm.name
from
firms
left join (
select future_job.*
from
(
select firmid, min(date) as mindate
from jobs
where date >= curdate()
group by firmid
) future
inner join jobs future_job
on future_job.firmid = future.firmid and future_job.date = future.mindate
union all
select past_job.*
from
(
select firmid, max(date) as maxdate
from jobs
group by firmid
having max(date) < curdate()
) past
inner join jobs past_job
on past_job.firmid = past.firmid and past_job.date = past.maxdate
) closest_job
on firms.id = closest_job.firmid

Answer 4

I think this does what I need:
select * from (
select firms.name, t2.closest_date from firms
left join
(
select * from (
--get first date in the future
SELECT firmid, MIN(date) closest_date
FROM jobs
WHERE date >= CURRENT_DATE
GROUP BY firmid
UNION ALL
-- most recent expired deadline
SELECT firmid, MAX(date)
FROM jobs
WHERE date < CURRENT_DATE
GROUP BY firmid) as t1
-- order so latest date is first
order by closest_date desc) as t2
on firms.id = t2.firmid
-- group by eliminates all but latest date
group by firms.id) as t3
order by closest_date asc;
Thanks for all the help on this