寻找作者人数

Question

有两个表：authors(authorid, name)和books(bookid, authorid, title, year)。

我可以编写哪种sql查询来计算books表中有多少作者，如果某个作者没有本书，则返回零？

我尝试过：

Select authors.name, count(authors.authorid) as count
FROM authors, books
where authors.authorid = books.authorid
group by authors.authorid;

示例输出：

name count
author1 1
author2 0

Answer 1

使用左联接和现代联接而不用逗号分隔联接

Select authors.name, count(books.authorid)
FROM authors left join books
on authors.authorid = books.authorid
 group by authors.name