这些数据集的内部连接与子查询的结果不同。

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');

上面两个表有外键关系，wcd.id = cases.wcd_id，让我们考虑与wcd.wid 5865相关的记录。结果应按理由_id分组，条件是max(cases.id)

我使用了下面的查询来实现，得到了预期的结果。

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;

结果。

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

用同样的方法查询5684，查询结果是 返回0行 虽然有数据可查.但我希望得到下面的行。

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

什么问题与查询，需要改变什么，以获得上述结果为5684。

这里是sqlfiddle链接

Answer 1

你需要回顾一下 wcd 表来进行适当的关联，因为你需要有 "最新 "的原因的行的id，每一个 wid - 而该列在 cases.

在MySQL 8.0中，我们只需要使用 row_number()...... 但你把你的问题标记为MySQL 5.6。我发现最简单的表达方式就是用相关的子查询。

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

然后你必须使用MIN并去掉Where子句，因为... ...

 ('162', '5684')
  ('161', '5684')

因为

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

看到 http:/sqlfiddle.com#！9fb456926