Question

表格例如：

event_id| group_id | updated_at
1 | 1 |2012-01-02
2 | 1 |2012-01-01
3 | 2 |2012-01-01
4 | 2 |2012-01-02
5 | 3 |2012-01-01

我需要选择所有具有最大日期的event_id，按group_id分组。所以正确的结果将是：

event_id| group_id | updated_at
1 | 1 |2012-01-02
4 | 2 |2012-01-02
5 | 3 |2012-01-01

我可以从选择MAX（updated_at）的同一行中选择event_id吗？

SELECT event_id, group_id, MAX(updated_at) 
FROM my_table 
GROUP BY group_id;

Answer 1

MySQL可能允许您建议的语法类型 - 指定非聚合列也不属于该组 - 但它是非标准的，恕我直言是一个坏习惯。

相反，你会这样做：

SELECT t1.event_id, t1.group_id, t1.updated_at
FROM my_table t1
INNER JOIN
(
    SELECT group_id, MAX(updated_at) AS max_updated_at
    FROM my_table
    GROUP BY group_id
) t2 ON t1.group_id = t2.group_id AND t1.updated_at = t2.max_updated_at

请注意，如果updated_at对于给定的group_id而言不是唯一的，则此查询将返回所有所述行。

Answer 2

与lc的解决方案类似，以下内容允许您获取最近一天的记录，然后获取那些记录中每天event_id最高的记录： -

SELECT t1.event_id, t1.group_id, t1.updated_at
FROM my_table t1
INNER JOIN
(
    SELECT group_id, MAX(updated_at) AS max_updated_at
    FROM my_table
    GROUP BY group_id
) t2 
ON t1.group_id = t2.group_id AND t1.updated_at = t2.max_updated_at
INNER JOIN
(
    SELECT group_id, updated_at, MAX(event_id) AS max_event_id
    FROM my_table
    GROUP BY group_id, updated_at
) t3 
ON t1.group_id = t3.group_id 
AND t1.event_id = t2.max_event_id
AND t3.max_updated_at = t3.updated_at

使用MAX功能从同一列中选择单元格

2 个答案: