Select last N rows following a condition - mysql

Question

I've got a database table with logs which has 3 columns:
date | status | projectId
status can be either 0 or 1, primary key is on date and projectID
I'm trying to find out how many times a projectID had status 0 since the last time it was 1.
so if there would be only one projectId
date | status | projectId
1 0 3
2 0 3
3 1 3
4 1 3
5 0 3
6 0 3
this should return 2 (row 5 and 6 are 0 and row 4 is 1)
The thing that makes it hard for me is that I have to maintain the order of date. What would be a good way to tackle such problems, and this one in particular?

Answer 1

Here is how you would do it for one project:
select count(*)
from logs l
where status = 0 and
projectid = 3 and
date > (select max(date) from logs where projectid = 3 and status = 1)
Here is how you would do it for all projects:
select l.projectId, count(l1.projectId)
from logs l left outer join
(select projectId, max(date) as maxdate
from logs
where status = 1
group by projectId
) l1
on l.projectId = l1.projectId and
l.date > l1.date and
l.status = 0
group by l.projectId;

Answer 2

here you have an option in just one select.
http://sqlfiddle.com/#!2/6ce87/11
select *
from logs
where status=0 and date > (select date from logs where status=1 order by date desc limit 1)

Answer 3

Here's one way to get the result for all project_id:
SELECT m.project_id
, COUNT(1) AS mycount
FROM ( SELECT l.project_id
, MAX(l.date) AS latest_date
FROM mytable l
WHERE l.status = 1
) m
JOIN mytable t
ON t.project_id = m.project_id
AND t.date > m.latest_date
AND t.status = 0
If you need only a subset of project_id, the predicate should be added to the WHERE clause in the inline view query:
WHERE l.status = 1
AND l.project_id IN (3,5,7)
EDIT
That query does not return a row if there is no status=0 row after the latest status=1 row. To return a zero count, this could be done with an outer join.
SELECT m.project_id
, COUNT(t.status) AS mycount
FROM ( SELECT l.project_id
, MAX(l.date) AS latest_date
FROM mytable l
WHERE l.status = 1
AND l.project_id IN (3)
) m
LEFT
JOIN mytable t
ON t.project_id = m.project_id
AND t.date > m.latest_date
AND t.status = 0
For optimum performance, the statement could make use of an index with leading columns of project_id and date (in that order) and including the status column, e.g.
ON mytable (`project_id`,`date`,`status`)