MySQL - 组中的条件选择

Question

我有一个包含几行的简单表格，我想按 id_room 对它们进行分组，并且仅当条件为真时才选择值。问题是即使有一行包含正确的日期列 year_month，条件也始终为假。

这是架构:

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);

这里查询:

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

这里是 SQL Fiddle

在 total 列中，我得到 0，我想得到值 3，因为 year_month 是 2016-10-01。为什么会这样？

Answer 1

您当然不需要那个CASE 条件并将该条件包含在您的WHERE 子句中而不是像

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;