Question

我有Users，Books和BookUser个表格。

UserID和UserName位于Users表中。 BookID和BookName位于Books表中。在BookUser表中，它合并了BookID和UserID，因此此BookUser表显示哪个用户已阅读哪本书。

我需要编写SQL查询来查找以下内容：

用户阅读的图书数量
阅读相同图书的其他用户的姓名
用户之间的普通书籍数量？

任何帮助都将不胜感激。

Answer 1

这是您为每个要求构建查询的基本方法：

--The number of books that a user reads
select count(bu.BookID) 
from BookUser bu 
join Users u on bu.UserID = u.UserID
where u.UserName = 'whatever'

--The name of other users who read the same books
select distinct u2.UserName
from Users u
join BookUser bu on u.UserID = bu.UserID
join BookUser bu2 on bu.BookID = bu2.BookID and bu.UserID <> bu2.UserID
join Users u2 on bu2.UserID = u2.UserID
where u.UserName = 'pick one'

--The number of common books between users
select distinct u.UserName, u2.UserName, count(*)
from Users u
join BookUser bu on u.UserID = bu.UserID
join BookUser bu2 on bu.BookID = bu2.BookID and bu.UserID <> bu2.UserID
join Users u2 on bu2.UserID = u2.UserID
group by u.UserName, u2.UserName

什么是用户名的SQL查询和常用书籍的数量？

1 个答案: