我有2个表,但链接在很多关系中,所以有3个表:
Table Author :
idAuthor,
Name
+----------+-------+
| idAuthor | Name |
+----------+-------+
| 1 | Renee |
| 2 | John |
| 3 | Bob |
| 4 | Bryan |
+----------+-------+
Table Publication:
idPublication,
Title,
Type,
Date,
Journal,
Conference
+---------------+--------------+------+-------------+------------+-----------+
| idPublication | Title | Date | Type | Conference | Journal |
+---------------+--------------+------+-------------+------------+-----------+
| 1 | Flower thing | 2008 | book | NULL | NULL |
| 2 | Bees | 2009 | article | NULL | Le Monde |
| 3 | Wasps | 2010 | inproceding | KDD | NULL |
| 4 | Whales | 2010 | inproceding | DPC | NULL |
| 5 | Lyon | 2011 | article | NULL | Le Figaro |
| 6 | Plants | 2012 | book | NULL | NULL |
+---------------+--------------+------+-------------+------------+-----------+
Table author_has_publication :
Author_idAuthor,
Publication_idPublication
+-----------------+---------------------------+
| Author_idAuthor | Publication_idPublication |
+-----------------+---------------------------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 1 | 5 |
| 2 | 5 |
| 3 | 5 |
| 3 | 6 |
+-----------------+---------------------------+
我想做的是让排名前X的作者拥有最多的出版物。
我使用此请求获得结果avec idAuthor拥有最多的出版物:
SELECT Author_idAuthor, COUNT(*) as count FROM Author_has_publication GROUP BY Author_idAuthor ORDER BY count DESC;
我得到了作者ID列表,按出版物数量排序:
+-----------------+-------+
| Author_idAuthor | count |
+-----------------+-------+
| 3 | 3 |
| 2 | 2 |
| 1 | 2 |
| 4 | 1 |
+-----------------+-------+
但是当我尝试选择与上一个查询的结果集的前X对应的作者时,我有一个错误
我正在尝试这个SELECT TOP 2 FROM author WHERE (SELECT Author_idAuthor, COUNT(*) as count FROM Author_has_publication GROUP BY Author_idAuthor ORDER BY count DESC)=idAuthor;
我想这可能是因为我的内部查询返回2行,我在这里做了一个简单的SELECT
或者我需要JOIN
,但我不知道如何在这里使用它。
答案 0 :(得分:1)
MySQL没有TOP关键字。但它有一个LIMIT关键字。无论如何,您的查询无效。
这里有几个选项。以下是相关子查询的示例:https://en.wikipedia.org/wiki/Correlated_subquery
SELECT
a.idAuthor,
a.Name ,
(SELECT COUNT(*) from author_has_publication ahp WHERE
ahp.Author_idAuthor = a.idAuthor) AS publication_count
FROM
author a
ORDER BY
publication_count DESC
LIMIT 2
正如引用的文章所指出的,上面的效率很低,因为需要为结果的每一行重新执行子查询。如果您实际上不需要结果集中的计数,则下面的效率会更高,因为子查询是非相关的并且只执行一次。
SELECT
a.idAuthor,
a.Name
FROM
author a
INNER JOIN
(select ahp.Author_idAuthor AS idAuthor, COUNT(*) as publication_count
FROM author_has_publication ahp GROUP BY ahp.Author_idAuthor LIMIT 2)
AS TEMP ON TEMP.idAuthor = a.idAuthor