inner join with subquery results differs for these data sets - mysql

Question

CREATE TABLE IF NOT EXISTS `wcd` (
`id` int(6) unsigned NOT NULL,
`wid` int(11) unsigned NOT NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `wcd` (`id`, `wid`) VALUES
('168', '5685'),
('167', '5685'),
('166', '5685'),
('165', '5685'),
('164', '5685'),
('163', '5685'),
('162', '5684'),
('161', '5684');
CREATE TABLE IF NOT EXISTS `cases` (
`id` int(6) unsigned NOT NULL,
`wcd_id` int(11) unsigned NOT NULL,
`reason_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `cases` (`id`, `wcd_id`, `reason_id`) VALUES
('20', '168', '4'),
('19', '168', '1'),
('18', '167', '6'),
('17', '167', '5'),
('16', '166', '4'),
('15', '166', '1'),
('14', '165', '4'),
('13', '165', '1'),
('12', '164', '1'),
('11', '163', '4'),
('10', '162', '1'),
('9', '162', '4'),
('8', '162', '5'),
('7', '161', '5'),
('6', '161', '6');
the above two table has foreignkey relation with wcd.id = cases.wcd_id,
Lets consider the records related to wcd.wid 5865. The result should be grouped by reason_id with the condition max(cases.id)
I used the query below to achieve this and got the result as expected.
SELECT d.id, d.wid, c.* FROM wcd d
LEFT JOIN cases c ON c.wcd_id = d.id
inner JOIN (SELECT MAX(id) AS max_id FROM cases GROUP BY reason_id) c2
ON c2.max_id = c.id
WHERE d.wid = 5685;
Result:
id wid id wcd_id reason_id
168 5685 19 168 1
168 5685 20 168 4
167 5685 17 167 5
167 5685 18 167 6
with the same query for 5684, the query returns 0 rows though there is data available for it. but I'm expecting the rows below.
id wid id wcd_id reason_id
162 5684 10 162 1
162 5684 9 162 4
162 5684 8 162 5
161 5684 6 161 6
What the issue with the query and what needs to be changed to get the result above for 5684.?
here is the sqlfiddle link

Answer 1

You need to look back at the wcd table to propery correlate, since you need the id of the row that has the "latest" reason per wid - and that column is not available in cases.
In MySQL 8.0, we would just use row_number()... but you tagged your question MySQL 5.6. I find that the simplest way to express this is with a correlated subquery:
SELECT d.id, d.wid, c.*
FROM wcd d
INNER JOIN cases c ON c.wcd_id = d.id
WHERE c.id = (
SELECT max(c2.id)
FROM wcd d2
INNER JOIN cases c2 ON c2.wcd_id = d2.id
WHERE d2.wid = d.wid AND c2.reason_id = c.reason_id
)
AND d.wid = 5685;

Answer 2

Then you must use MIN and get rid of the Where Clause.because
('162', '5684')
('161', '5684')
because
SELECT
d.id
, d.wid
,
c.*
FROM
wcd d
LEFT JOIN
cases c
ON c.wcd_id = d.id
inner JOIN (SELECT MIN(id) AS min_id FROM cases GROUP BY reason_id) c2
ON c2.min_id = c.id
see http://sqlfiddle.com/#!9/fb4569/26