1

这是我的项目表：

id (int)
category_id (int)
title (varchar 255)
updated_timestamp (int)

每个项目都有一个 category_id。假设我有这些数据：

1  |  14  |  item 1  |  1376164492
2  |  11  |  item 2  |  1376164508
3  |  12  |  item 3  |  1376164512
4  |  14  |  item 4  |  1376164532
5  |  12  |  item 5  |  1376164542
6  |  11  |  item 6  |  1376164552

我的目标是获取每个类别的最后更新时间戳。结果我需要这个：

11 = 1376164522 (greatest value for all items with category_id = 11)
12 = 1376164542 (greatest value for all items with category_id = 12)
14 = 1376164532 (greatest value for all items with category_id = 14)

这是我的 SQL：

SELECT 
    `category_id`, `updated_timestamp` 
FROM 
    `my_table` 
GROUP BY 
    `category_id` 
ORDER BY 
    `updated_timestamp` 
DESC

但是当我运行它时，我会得到这样的结果：

11 = 1376164508 (lowest value for all items with category_id = 11)
12 = 1376164512 (greatest value for all items with category_id = 12)
14 = 1376164492 (greatest value for all items with category_id = 14)

换句话说，DESC 不起作用。我做错了什么？

谢谢！

4

3 回答 3

2

此查询允许您获取表中的所有列。

SELECT  a.*
FROM    items a
        INNER JOIN
        (
            SELECT  category_id, MAX(updated_timestamp) updated_timestamp
            FROM    items
            GROUP   BY category_id
        ) b ON a.category_id = b.category_id AND
                a.updated_timestamp = b.updated_timestamp
于 2013-08-21T04:38:50.150 回答
2 
SELECT *
FROM `my_table` a
WHERE NOT EXISTS (
    SELECT 1 FROM `my_table` b
    WHERE b.`category_id` = a.`category_id`
    AND b.`updated_timestamp` > a.`updated_timestamp`
)
ORDER BY `updated_timestamp`
于 2013-08-21T04:39:54.893 回答
2

MAX(通过添加updated_timestamp尝试如下查询)

SELECT 
    `category_id`, MAX(`updated_timestamp` )
FROM 
    `Table1` 
GROUP BY 
    `category_id` 
ORDER BY 
    `updated_timestamp` 
DESC

SQL 小提琴演示

于 2013-08-21T04:40:11.990 回答