I need to sovle this problem:
I have a subject table:
+-------------+--------------+------+-----+---------------------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------------------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| main_title | varchar(30) | NO | | "" | |
| sub_title | varchar(30) | NO | | "" | |
| type | tinyint(4) | NO | | 1 | |
| thumbnail | int(11) | NO | | 0 | |
| main_pic | int(11) | NO | | 0 | |
| info_url | varchar(128) | NO | | "" | |
| create_time | timestamp | NO | | CURRENT_TIMESTAMP | |
| modify_time | timestamp | NO | | 0000-00-00 00:00:00 | |
| status | tinyint(4) | NO | | 1 | |
+-------------+--------------+------+-----+---------------------+----------------+
a news table:
+---------------+---------------+------+-----+---------------------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+---------------+------+-----+---------------------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| title | varchar(60) | NO | | | |
| thumbnail | int(11) | NO | | 0 | |
| abstract | varchar(2048) | NO | | | |
| paragraphs | text | NO | | NULL | |
| about_subject | varchar(128) | NO | | | |
| pv | int(11) | NO | | 0 | |
| share_count | int(11) | NO | | 0 | |
| like_count | int(11) | NO | | 0 | |
| comment_count | int(11) | NO | | 0 | |
| source | varchar(64) | NO | | | |
| source_url | varchar(128) | NO | UNI | | |
| signature | varchar(40) | NO | | | |
| creator_id | int(11) | NO | | 0 | |
| create_time | timestamp | NO | | CURRENT_TIMESTAMP | |
| publish_time | timestamp | NO | | 0000-00-00 00:00:00 | |
| editor_id | int(11) | NO | | 0 | |
| modify_time | timestamp | NO | | 0000-00-00 00:00:00 | |
| status | tinyint(4) | NO | MUL | 0 | |
+---------------+---------------+------+-----+---------------------+----------------+
Now , I want to get the top 20 newest news for every subject at one query.I have tried:
SELECT
concat('3','_',news.id) as id, 3 as target_type,
news.id as target_id, news.title as target_title,
news.abstract as target_abstract, news.paragraphs as target_paragraphs,
news.source as target_source, news.pv as target_pv,
news.like_count as target_like_count, news.comment_count as target_comment_count, news.publish_time as target_create_time,
subject.id as subject_id, subject.main_title as subject_name
FROM
subject
LEFT JOIN news ON subject.id = news.about_subject
ORDER BY news.publish_time DESC
-- LIMIT 0 ,20
How can I get the top 20 for each subject and order all the data by subject.id?
答案 0 :(得分：0)
How to select the first/least/max row per group in SQL aritcle有很多方法，我只选择一个。
set @num = 0,@subject_id = '';
SELECT
concat('3','_',news_inner.id) as id, 3 as target_type, news_inner.id as target_id,
news_inner.title as target_title, news_inner.abstract as target_abstract,
news_inner.paragraphs as target_paragraphs, news_inner.source as target_source,
news_inner.pv as target_pv, news_inner.like_count as target_like_count,
news_inner.comment_count as target_comment_count, news_inner.publish_time as target_create_time
FROM
(
select
id, about_subject, title, abstract, paragraphs, source, pv,
like_count, comment_count, publish_time,
@num := if(@subject_id = about_subject, @num + 1, 1) as row_number,
@subject_id := about_subject
from news
order by about_subject DESC,publish_time DESC
) as news_inner
where news_inner.row_number <= 20