MySQL - conditional select in group - mysql

Question

I have a simple table with several rows and I would like to group them by id_room and select value only when condition is true. Problem is that condition is always false even there is a row with right date column year_month.
Here is schema:
CREATE TABLE tbl_account_room (
`id` int,
`year_month` date,
`value` int,
`id_room` int
);
INSERT INTO tbl_account_room
(`id`, `year_month`, `value`, `id_room`)
VALUES
(1, '2016-08-01', 1, 300),
(2, '2016-09-01', 2, 300),
(3, '2016-10-01', 3, 300);
and here query:
SELECT
(case when '2016-10-01' = ar.year_month then ar.value else 0 end) as total
FROM tbl_account_room AS ar
WHERE ar.year_month >= "2016-08-01"
AND ar.year_month <= "2016-11-01"
and ar.id_room = '300'
GROUP BY ar.id_room
LIMIT 10
Here is SQL Fiddle
In column total I'm getting 0 and I would like to get value 3 because year_month is 2016-10-01. Why this happens?

Answer 1

You certainly don't need that CASE condition and include that condition in your WHERE clause rather like
SELECT
ar.value as total,
GROUP_CONCAT(ar.year_month)
FROM tbl_account_room AS ar
WHERE ar.year_month = '2016-10-01'
GROUP BY ar.id_room;

Answer 2

Not sure why you want the result like that, here you can use self join to do that:
SELECT
MAX(t1.value) as total,
GROUP_CONCAT(ar.year_month)
FROM tbl_account_room AS ar
LEFT JOIN tbl_account_room AS t1
ON ar.id_room = t1.id_room
AND ar.year_month = t1.year_month
AND t1.year_month = '2016-10-01'
WHERE ar.year_month >= "2016-08-01"
AND ar.year_month <= "2016-11-01"
and ar.id_room = '300'
GROUP BY ar.id_room
LIMIT 10;
and here is SQLFiddle Demo.