MySQL 按结果排序

Question

我有以下表格结构：

table_1

id     title
------------
155    item1
156    item2
157    item3

table_2

id     model
------------
155    100
156    100
157    200

table_3

id     cid
------------
155    5
156    5
157    5

我想按模型（来自table_2）对我的结果进行分组和确保它returns最高 ID（按 id 降序排列）。

我尝试了以下方法，但 order by 子句似乎不起作用：

SELECT a.id, b.model FROM table_1 as a
INNER JOIN table_2 as b on b.id = a.id
INNER JOIN table_3 as c on c.id = a.id
WHERE c.cid = 5
GROUP BY b.model
ORDER BY a.id desc

我做错了什么？

Answer 1

你可以这样做

select 
t1.id, 
t2.model 
from table_1 t1 join table_3 t3 on t3.id = t1.id 
join ( 
  select max(id) as id , model from table_2 group by model 
)t2 on t1.id=t2.id 
where t3.cid = 5;

Answer 2

SELECT max(a.id), b.model FROM table_1 as a
INNER JOIN table_2 as b on b.id = a.id
INNER JOIN table_3 as c on c.id = a.id
WHERE c.cid = 5
GROUP BY b.model
ORDER BY max(a.id) desc

此处演示：http://sqlfiddle.com/#!9/afb0b/2

此方法有效而您的初始尝试无效的原因是 GROUP BY 子句中不存在或未在聚合函数（例如 MAX）中使用的任何字段都将具有不确定的值（即，mysql 引擎不保证您将从组中获得哪个值）。

MySQL GROUP BY Handling

The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate.

事实上，MySQL 是为数不多的（唯一？）sql 数据库之一，可以让您在 select 子句中放置非聚合、非分组字段而不会出错.

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