我需要为每个不同的记录选择几行。几乎就像这里问的那样在 SQL Server 2008 中为每个不同 ID 选择前 n 条记录 https://stackoverflow.com/questions/3364224/select-first-n-records-for-each-distinct-id-in-sql-server-2008,虽然我使用MySQL。
在这种情况下,可以通过运行 21 个查询来实现目的:1 个常规查询和 20 个查询来获取子记录,即如下所示:
SELECT DISTINCT `user_id`
FROM `posts`
WHERE `deleted` = '0'
ORDER BY `user_id` ASC
LIMIT 20
...选择所需的所有行,然后
SELECT *
FROM `posts`
WHERE `deleted` = '0'
AND `user_id` = ?
ORDER BY `id` DESC
LIMIT 5
...在第一个查询选择的每一行的循环中。
基本上,我需要获取每个用户的 5 个帖子。我需要在单个查询中完成此操作。这posts
设置只是一个例子,我做了这个,所以希望它更容易理解我的需要。
我从以下查询开始:
SELECT *
FROM `posts`
WHERE `user_id`
IN (
SELECT DISTINCT `user_id`
FROM `posts`
ORDER BY `user_id` DESC
LIMIT 4
)
LIMIT 5
但我明白了#1235 - This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
error.
所以我尝试过JOIN
像建议的想法here https://stackoverflow.com/a/2856430/722036:
SELECT posts.id,
posts.user_id,
NULL
FROM (
SELECT posts.user_id
FROM posts
WHERE posts.deleted = 0
LIMIT 20
) q
JOIN posts
ON posts.user_id = q.user_id
我还按照建议尝试了几个嵌套查询here https://stackoverflow.com/a/7124492/722036:
SELECT *
FROM posts
WHERE user_id IN (
SELECT * FROM (
SELECT user_id
FROM posts
LIMIT 20
)
as t);
以及网上找到的其他解决方案。但它们要么不起作用,要么只是简单地从数据库中选择前 N 行(无论条件和出于某种原因的连接如何)。尝试过LEFT JOIN
, RIGHT JOIN
, even INNER JOIN
,但仍然没有成功。
请帮忙。
UPDATE忘了说该表的大小约为 5GB。
UPDATE尝试了子子查询:
SELECT *
FROM `posts`
WHERE
`user_id` IN ( SELECT `user_id` FROM (
SELECT DISTINCT `user_id`
FROM `posts`
ORDER BY `user_id` DESC
LIMIT 4 ) limit_users
)
LIMIT 5
与上面相同,它返回以下内容:
+----+---------+------+
| id | user_id | post |
+----+---------+------+
| 1 | 1 | a |
+----+---------+------+
| 2 | 1 | b |
+----+---------+------+
| 3 | 1 | c |
+----+---------+------+
| .. | .. | .. |
IE。 5(这是外部的LIMIT
设置为)同一用户的行。奇怪的是,如果我单独运行子查询和子子查询:
SELECT `user_id` FROM (
SELECT DISTINCT `user_id`
FROM `posts`
ORDER BY `user_id` DESC
LIMIT 4 ) limit_users
我得到 4 个不同的值:
+---------+
| user_id |
+---------+
| 1 |
+---------+
| 2 |
+---------+
| 3 |
+---------+
| 4 |
+---------+