MySQL: select row only where closest to date has column value - mysql

Question

I want to return all rows that were public in May (2019-05), so if a row was turned to draft (and not back to public) at any point before the end of May, I don't want it. For example:
id | post_id | status | date
-------------------------
1 | 1 | draft | 2019-03-25
2 | 1 | public | 2019-04-02
3 | 1 | draft | 2019-05-25
4 | 2 | draft | 2019-03-10
5 | 2 | public | 2019-04-01
6 | 2 | draft | 2019-06-01
The desired result for the above would return post_id 2 because its last status change prior to the end of May was to public.
post_id 1 was put back in draft before the end of May, so it would not be included.
I'm not sure how to use the correct join or sub-queries to do this as efficiently as possible.

Answer 1

You seem to want the status as of 2019-05-31. A correlated subquery seems like the simplest solution:
select t.*
from t
where t.date = (select max(t2.date)
from t t2
where t2.post_id = t.post_id and
t2.date <= '2019-05-31'
);
To get the ones that are public, just add a WHERE condition:
select t.*
from t
where t.date = (select max(t2.date)
from t t2
where t2.post_id = t.post_id and
t2.date <= '2019-05-31'
) and
t.status = 'public';
For performance, you want an index on (post_id, date).
You can also phrase this using a JOIN:
select t.*
from t join
(select t2.post_id, max(t2.date) as max_date
from t t2
where t2.date <= '2019-05-31'
group by t2.post_id
) t2
on t2.max_date = t.date
where t.status = 'public';
I would expect the correlated subquery to have better performance with the right indexes. However, sometimes MySQL surprises me.

Answer 2

we need to determine whether
the status of each post_id is public prior to the month May (the subquery with max(date)),
any post_id exists with status not equals public within the month May,
and then exclude the post_id satisfying the matter 2.
So, you can use :
select distinct t1.post_id
from tab t1
where t1.post_id not in
(
select distinct t1.post_id
from tab t1
join
(
select post_id, max(date) as date
from tab
where '2019-05-01'> date
group by post_id ) t2
on t1.post_id = t2.post_id
where t1.status != 'public'
and t1.date < '2019-06-01'
and t1.date > '2019-04-30'
);
+---------+
| POST_ID |
+---------+
| 2 |
+---------+
Demo