MySQL - 在视图中获取不同的行

Question

好的，在这里。我试图让一个观点，即表明这一点：

+---------+---------+-------+--------+------------+---------------------+---------------+-------------------+-------------+ 
| 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) 

是这样做的查询是：

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，因为它有一个嵌套查询，我不能用它作为视图。目前我能想到的最好的查询工作的看法是这样的：

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 

但是，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POST_ID一排，我希望它是最新的一个。有没有人有什么建议？在处理软删除等问题时，我正在使用视图来尝试并使生活更轻松，并且从理论上讲，还需要从长远角度查询我想要的数据。

非常感谢！

Answer 1

会是这样的任何用途？我不知道这件事，因为它包含一个子查询，这是给你的麻烦，但在不同的地方......

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); 

性能可能不是什么大书特书，无论是。

Answer 2

您可以使用

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

只得到最新的行。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