Question

我想创建一个SQL语句，告诉我有多少本书由特定作者撰写。

当您知道authorid（books表的外键）时，这很简单：

select count(*) from books where authorid = 25;
15

你可以看到id为15的作者写了15本书。是否可以为所有作者创建一个声明，使得输出如下？

author_id, author_name, number_of_books
1          Michael      15
2          Robin        7
...

Answer 1

您可以使用group by子句执行此操作：

select 
    a.author_id,
    a.author_name,
    count(*) as number_of_books
from
    authors a inner join
    books b on b.author_id = a.author_id
group by
    a.author_id,
    a.author_name
order by 
    number_of_books

Answer 2

这将是GROUP BY查询：

select author_id, author_name, count(*) as number_of_books 
   from books
   join author on books.author_id = author.id 
   group by author_id, author_name;

Answer 3

SELECT
    books.author_id, authors.author_name, COUNT(books.author_id) AS number_of_books
FROM
    authors INNER JOIN books ON books.author_id = authors.id
GROUP BY
    author_name;

另外，请确保books.author_id列不为NULL，否则性能将受到影响。

从SQLite中的特定外键中选择计数

3 个答案: