给定两个相关表，如何确定最常见的关系？

Question

给定 3 个表：用户、书籍、book_users，我如何确定哪些是公共书籍？

users: id, first_name, last_name
books: id, name
books_users: book_id, user_id

设计器输出，类似于：

book | count
radBookName | 22
SemiRad | 22

谢谢

Answer 1

您似乎想要带有 GROUP BY 子句的简单 JOIN :

SELECT b.name, count(*) as user_count
FROM books b INNER JOIN
     books_users bu
     ON bu.book_id = b.id
GROUP BY b.name;

如果一本书有相同的用户，这将产生重复计数，如果您想要唯一计数，请改用 count(distinct bu.user_id)。

Given two related tables, how to determine the most common relationships?