MySQL —在视图中获取不同的行

Question

好，去 我正在尝试显示一个视图：

+---------+---------+-------+--------+------------+---------------------+---------------+-------------------+-------------+
| post_id | status  | title | body   | ip_address | time_stamp          | category_name | sub_category_name | post_type   |
+---------+---------+-------+--------+------------+---------------------+---------------+-------------------+-------------+
|       1 | enabled | test  | test 2 |            | 2010-05-20 01:22:17 | For Sale      | Computers         | transaction |
+---------+---------+-------+--------+------------+---------------------+---------------+-------------------+-------------+
1 row in set (0.00 sec)

执行此操作的查询是：

SELECT post.id AS post_id, 
post.status AS status, 
post_data.title AS title, 
post_data.body AS body, 
post_data.ip_address AS ip_address, 
post_data.time_stamp AS time_stamp, 
post_category.name AS category_name, 
post_sub_category.name AS sub_category_name, 
post_category.type AS post_type 
FROM post,
(
SELECT * FROM post_data WHERE post_data.post_id = post_id ORDER BY post_data.post_id DESC LIMIT 1
) AS post_data,
post_sub_category, 
post_category 
WHERE 
post.sub_category_id = post_sub_category.id AND 
post_sub_category.category_id = post_category.id

但是，由于它具有嵌套查询，因此无法将其用作视图。 目前，我能想到的最好的查询视图是：

SELECT
post.id AS post_id, 
post.status AS status, 
post_data.title AS title, 
post_data.body AS body, 
post_data.ip_address AS ip_address, 
post_data.time_stamp AS time_stamp, 
post_category.name AS category_name, 
post_sub_category.name AS sub_category_name, 
post_category.type AS post_type 
FROM post,
post_data,
post_sub_category, 
post_category 
WHERE 
post.sub_category_id = post_sub_category.id AND 
post_sub_category.category_id = post_category.id
ORDER BY post_data.id DESC

但这只是返回：

+---------+---------+-------+-----------+----------------+---------------------+---------------+-------------------+-------------+
| post_id | status  | title | body      | ip_address     | time_stamp          | category_name | sub_category_name | post_type   |
+---------+---------+-------+-----------+----------------+---------------------+---------------+-------------------+-------------+
|       1 | enabled | test  | test 2    |                | 2010-05-20 01:22:17 | For Sale      | Computers         | transaction |
|       1 | enabled | TEST  | TEST BODY | 192.168.10.155 | 2010-05-19 23:09:15 | For Sale      | Computers         | transaction |
+---------+---------+-------+-----------+----------------+---------------------+---------------+-------------------+-------------+
2 rows in set (0.00 sec)

我只希望每个post_id有一行，并且我希望它是最新的。 有没有人有什么建议？ 在处理软删除和诸如此类的事情时，我使用视图来尝试使生活变得更轻松，并且从长远来看，从理论上讲，这也使查询我想要的数据变得更加轻松。

非常感谢！

Answer 1

您可以使用

 GROUP BY post_data.post_id HAVING MAX(post_data.time_stamp)

仅获取最新行。 因此，完整的查询可能如下所示：

SELECT
post.id AS post_id, 
post.status AS status, 
post_data.title AS title, 
post_data.body AS body, 
post_data.ip_address AS ip_address, 
post_data.time_stamp AS time_stamp, 
post_category.name AS category_name, 
post_sub_category.name AS sub_category_name, 
post_category.type AS post_type 
FROM post,
post_data,
post_sub_category, 
post_category 
WHERE 
post.sub_category_id = post_sub_category.id AND 
post_sub_category.category_id = post_category.id
GROUP BY post_data.post_id HAVING MAX(post_data.time_stamp)
ORDER BY post_data.id DESC

Answer 2

这样的东西有用吗？ 我不确定，因为它包含一个子查询，这给您带来麻烦，但是在另一个地方...

SELECT post.id AS post_id, 
   post.status AS status, 
   post_data.title AS title, 
   post_data.body AS body, 
   post_data.ip_address AS ip_address, 
   post_data.time_stamp AS time_stamp, 
   post_category.name AS category_name, 
   post_sub_category.name AS sub_category_name, 
   post_category.type AS post_type 
FROM post,
     post_data,
     post_sub_category, 
     post_category 
WHERE post.sub_category_id          = post_sub_category.id AND 
      post_sub_category.category_id = post_category.id     AND
      post_data.post_id             = post_id              AND
      post_data.time_stamp = (SELECT MAX(time_stamp)
                                  FROM post_data
                                  WHERE post_data.post_id = post_id);

性能可能也不是值得一提的。