per - sql top 1 from each group

如何選擇每組的前N行? (2)

您可以使用相關的子查詢來進行計數：

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;

對於一個小型數據庫，這應該沒問題。如果您在Book(AuthorId, BookId)上創建一個複合索引Book(AuthorId, BookId)那麼這將有助於查詢。

我有兩個這樣的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

我想做一個SELECT查詢，將返回每個AuthorId的前N（例如兩個）行，按標題排序（“選擇每個作者的前兩本書”）。

示例輸出：

 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

我怎樣才能建立這個查詢？

（是的，我發現了一個類似的話題，而且我知道如何只返回一行（第一或最上面），問題在於兩者）。

還有另一種變體：

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