sql - sql - 三表连接

Question

表结构（效率不高，但我必须使用它，而且我无法更改它。）：

大学表 - UniversityName、UniversityId

BookLease 表 - BookId、UniversityId、LeaseDate

图书表 - BookId、UniversityId、类别、Page_Count。

我必须找到名称为“XYZ”的大学到目前为止已阅读/租用的总页数。这是我到目前为止所拥有的：

select sum(bookTable.Page_count) 
from University u
join (select bl.UniversityId AS universityId, b.page_count as Counter
       BookLease bl
       join Book bk
            on bl.BookId = bk.BookId) as bookTable
on  
     bookTable.universityId = u.UniversityId
where
     u.Name = "XYZ"

这似乎是错误和低效的。是吗？有没有更好的方法来写这个？

Answer 1

你可以这样做：

select 
   sum(b.page_count) as pages_read
   , count(bl.bookid) as no_leased
from university u 
   inner join book b on b.universityid = u.universityid
   inner join booklease bl on bl.bookid = b.bookid
                           and bl.universityid = u.universityid
where u.name = 'XYZ'
and bl.lease_date > [some lease date]

Answer 2

只需直接加入表，而不是子查询：

select
  sum(bk.Page_count) 
from
  University u
  inner join BookLease bl on bl.UniversityId = u.UniversityId
  inner join Book bk on bk.BookId = bl.BookId
where
  u.Name = "XYZ"