MySQL Query for finding a "LAST" row, based on two fields - mysql

Question

I have the following MySQL table to log the registration status changes of pupils:
CREATE TABLE `pupil_registration_statuses` (
`status_id` INT(11) NOT NULL AUTO_INCREMENT,
`status_pupil_id` INT(10) UNSIGNED NOT NULL,
`status_status_id` INT(10) UNSIGNED NOT NULL,
`status_effectivedate` DATE NOT NULL,
PRIMARY KEY (`status_id`),
INDEX `status_pupil_id` (`status_pupil_id`)
)
COLLATE='utf8_general_ci'
ENGINE=MyISAM;
Example data:
INSERT INTO `pupil_registration_statuses` (`status_id`, `status_pupil_id`, `status_status_id`, `status_effectivedate`) VALUES
(1, 123, 1, '2013-05-06'),
(2, 123, 2, '2014-03-15'),
(3, 123, 5, '2013-03-15'),
(4, 123, 6, '2013-05-06'),
(5, 234, 2, '2013-02-02'),
(6, 234, 4, '2013-04-17'),
(7, 345, 2, '2014-02-01'),
(8, 345, 3, '2013-06-01');
It is possible that statuses can be inserted, thus the sequence of dates does not necessarily follow the same sequence of IDs.
For example: status_id 1 might has a date of 2013-05-06, but status_id 3 might have a date of 2013-03-15.
status_id values are, however, sequential within any particular date. Thus if a pupil's registration status changes multiple times on one day then the last row will will reflect their status for that date.
It is necessary to find out a particular student's registration status on a particular date. The following query works for an individual pupil:
SELECT *
FROM pupil_registration_statuses
WHERE status_pupil_id = 123
AND status_effectivedate <= '2013-05-06'
ORDER BY status_effectivedate DESC, status_id DESC
LIMIT 1;
This returns the expected row of status_id = 4
However, I now need to issue a (single) query to return the status for all pupils on a particular date.
The following query is proposed, but doesn't obey the "last status_id in a day" requirement:
SELECT *
FROM pupil_registration_statuses prs
INNER JOIN (SELECT status_pupil_id, MAX(status_effectivedate) last_date
FROM pupil_registration_statuses
WHERE status_effectivedate <= '2013-05-06'
GROUP BY status_pupil_id) qprs ON prs.status_pupil_id = qprs.status_pupil_id AND prs.status_effectivedate = qprs.last_date;
This query, however, returns 2 rows for pupil 123.
EDIT
To clarify, if the input is the date '2013-05-06', I expect to get the rows 4 and 6 from the query.
http://sqlfiddle.com/#!2/68ee6/2

Answer 1

Is this what you're after?
SELECT a.*
FROM pupil_registration_statuses a
JOIN
( SELECT prs.status_pupil_id
, MIN(prs.status_id) min_status_id
FROM pupil_registration_statuses prs
JOIN
( SELECT status_pupil_id
, MAX(status_effectivedate) last_date
FROM pupil_registration_statuses
WHERE status_effectivedate <= '2013-05-06'
GROUP
BY status_pupil_id
) qprs
ON prs.status_pupil_id = qprs.status_pupil_id
AND prs.status_effectivedate = qprs.last_date
GROUP
BY prs.status_pupil_id
) b
ON b.min_status_id = a.status_id;
http://sqlfiddle.com/#!2/68ee6/7
(Incidentally, there's an ugly and undocumented hack for this kind of problem which goes something like this:
SELECT x.* FROM (SELECT * FROM prs WHERE status_effectivedate <= '2013-05-06' ORDER BY status_pupil_id, status_effectivedate DESC, status_id)x GROUP BY status_pupil_id;
...but I didn't tell you that! ;) )

Answer 2

If I understood right, you want to...
1) Get 1 row per person.
2) Get the status changes from the specific day you manually input.
3) Get the last status changes from within the specific day.
If that's right, you need the query you already have ordering by date and then by id, just with a distinct.
SELECT DISTINCT on status_pupil_id *
FROM pupil_registration_statuses
WHERE status_pupil_id = 123
AND status_effectivedate <= '2013-05-06'
ORDER BY status_effectivedate DESC, status_id DESC

Answer 3

I have changed where clause, please try it.
SELECT *
FROM pupil_registration_statuses prs
INNER JOIN (SELECT status_pupil_id, MAX(status_effectivedate) last_date
FROM pupil_registration_statuses
WHERE Datediff(status_effectivedate, '2013-05-06') <= 0
GROUP BY status_pupil_id) qprs ON prs.status_pupil_id = qprs.status_pupil_id AND prs.status_effectivedate = qprs.last_date;
EDIT
Try this
SELECT *
FROM
(
select status_pupil_id,max(status_id) as status_id from pupil_registration_statuses innr
--where Datediff(dd,status_effectivedate, '2013-05-06') >= 0
group by status_pupil_id
)as ca
inner join pupil_registration_statuses prs on prs.status_id = ca.status_id
where Datediff(dd,prs.status_effectivedate, '2013-05-06') >= 0