请问这MySQL quearly select 所有写书的作者？

Question

我在尝试解决这个问题时遇到了很多麻烦

Write a query to show the number of authors who have written a book

Author(AuthorID, AuthorName, Address, TelephoneNo, PublisherCode)

Book (BookID, Name, ReleaseDate, Price, AuthorID)

我有

SELECT a.AuthorName, COUNT(b.*) AS ‘number of books written’
FROM Author a JOIN Book b ON  a.AuthorID = b.BookID 
GROUP BY a.AuthorName;

计算每个作者所写书籍的数量。

这不是我知道的正确的，但我想不通？？

Answer 1

你们非常接近。您需要加入作者 ID。您目前混用了作者 ID 和图书 ID，这不会正确匹配。

SELECT 
   a.AuthorName, 
   COUNT(b.*) AS ‘number of books written’
FROM Author a 
JOIN Book b ON a.AuthorID = b.AuthorID 
GROUP BY a.AuthorName;

Answer 2

如果您只想得到一个数字，表示至少在以下查询中使用书籍的作者总数

select count(*) as author_count from Author where exists (select 1 from Book where Book.AuthorID = Author.AuthorID)

Answer 3

假设要求计算至少拥有一本书的作者，满足该要求的最简单查询是：

SELECT COUNT(DISTINCT b.authorid)
  FROM book b

我们可能想为返回的列分配一个别名（名称）：

SELECT COUNT(DISTINCT b.authorid) AS `count_of_authors_who_have_at_least_one_book`
  FROM book b

我们也可以对 author table 进行连接，但这里没有必要，除非 book authorid table 未出现在 author table 中（即没有外键约束，或者未强制执行参照完整性）

获取拥有两本书或更多书籍的作者的查询会稍微复杂一些：

SELECT COUNT(*)
  FROM ( -- authors of two or more books 
         SELECT b.authorid
           FROM book b
          GROUP
             BY b.authorid
         HAVING COUNT(1) >= 2  
       ) c

如果我们希望作者只有一本书（而不是两本或更多），我们可以调整 HAVING 子句中的条件：

SELECT COUNT(*) AS `count_authors_of_exactly_one_book`
  FROM ( -- authors of exactly one book
         SELECT b.authorid
           FROM book b
          GROUP
             BY b.authorid
         HAVING COUNT(1) = 1  
       ) c

请问这MySQL quearly select 所有写书的作者？

Will this MySQL quearly select all of the authors who wrote a book?

mysql

relational-database