How to select the first N rows of each group?

Question

I have two SQLite tables like this:

 AuthorId | AuthorName
----------------------
 1        | Alice
 2        | Bob
 3        | Carol
 ...      | ....


 Boo         


        

Answer 1

There is alternative variant:

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

Answer 2

You can do the counting using a correlated subquery:

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;

For a small database this should be fine. If you create a composite index on Book(AuthorId, BookId) then that will help the query.

Answer 3

Here you go. Might be too late but I just saw the post. You can change the <=2 to match the n you need.

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