我有两个类似这样的SQLite表:
AuthorId | AuthorName
----------------------
1 | Alice
2 | Bob
3 | Carol
... | ....
BookId | AuthorId | Title
----------------------------------
1 | 1 | aaa1
2 | 1 | aaa2
3 | 1 | aaa3
4 | 2 | ddd1
5 | 2 | ddd2
... | ... | ...
19 | 3 | fff1
20 | 3 | fff2
21 | 3 | fff3
22 | 3 | fff4
我想做一个SELECT查询,返回每个AuthorId的前N行(例如两行),按标题排序("选择每个作者的前两本书")。
样本输出:
BookId | AuthorId | AuthorName | Title
------------------------------------------
1 | 1 | Alice | aaa1
2 | 1 | Alice | aaa1
4 | 2 | Bob | ddd1
5 | 2 | Bob | ddd2
19 | 3 | Carol | fff1
20 | 3 | Carol | fff2
如何构建此查询?
(是的,我发现了一个类似的主题,我知道如何只返回一行(第一行或顶部)。问题出在两者身上)。
您可以使用相关子查询进行计数:
SELECT b.BookId, a.AuthorId, a.AuthorName, b.Title
FROM Author a join
Book b
on a.AuthorId = b.AuthorId
where (select count(*)
from book b2
where b2.bookId <= b.BookId and b2.AuthorId = b.AuthorId
) <= 2;
对于小型数据库来说,这应该没问题。如果您在Book(AuthorId, BookId)
上创建一个复合索引,那么这将有助于查询。
还有另一种变体:
SELECT * FROM (
SELECT * FROM BOOK, AUTHOR
WHERE BOOK.AUTHORID = AUTHOR.AUTHORID
) T1
WHERE T1.BOOKID IN (
SELECT T2.BOOKID FROM BOOK T2
WHERE T2.AUTHORID = T1.AUTHORID
ORDER BY T2.BOOKTITLE
LIMIT 2
)
ORDER BY T1.BOOKTITLE
开始吧。可能太晚了,但我刚刚看到帖子。您可以更改<2来匹配所需的n。
SELECT
a.authorid,
a.authorname,
b.bookid,
b.booktitle
FROM author a
JOIN book b ON b.authorid = b.authorid
QUALIFY ROW_NUMBER() OVER (PARTITION BY a.authorid
ORDER BY b.booktitle ASC) <=2