为作者相关的书算错了为什么以及如何解决？

问题描述

我正在使用sql Server 2012，但遇到一个问题：与作者有关​​的书计数错误。为什么艾哈迈德（Ahmed）认为与作者有关​​的书籍计数错误？

当计数作者ahmed我得到一个错误的结果时，尽管它在books表中的相关记录为3，而bookupdate 3则为6。

请问如何解决？

create table #books  
(  
    BookId int,BookName nvarchar(200),AuthorId  int  
)  

create table #booksUpdate  
(  
    BookId int,AuthorId  int  
)  

insert into #booksUpdate  
values (119,'matlab',1),(120,'3dmax',(121,'c',1)  

create table #Authors  
(  
    AuthorId int,AuthorName nvarchar(200)  
)  

insert into #Authors  
values  (1,'Ahmed'),(2,'Mohamed'),(3,'Eslam')  

insert into #books  
values (122,'c#',(233,'Java',(555,'c++',(666,'photoshop',2),(777,'asp.net',(888,'python',2)  
  
select 
    a.authorName,count(b.BookName) as countBooks,count(bu.BookName) as countBooksUpdate,(count(bu.BookName) + count(b.BookName)) as Total 
from 
    #Authors a   
left join 
    #books b on a.AuthorId = b.AuthorId  
left join 
    #booksUpdate bu on a.AuthorId = bu.AuthorId  
group by 
    a.authorName

输出：

authorName  countBooks  countBooksUpdate    Total  
--------------------------------------------------
Ahmed            9              9             18  
Eslam            0              0              0  
Mohamed          3              0              3
 

作者Ahmed的结果是错误的。

应该是

authorName  countBooks  countBooksUpdate    Total 
-------------------------------------------------
Ahmed            3                 3          6  

如何修复查询以返回正确的结果？

预期结果是

authorName  countBooks  countBooksUpdate    Total  
-------------------------------------------------
Ahmed           3             3                6  
Eslam           0             0                0  
Mohamed         3             0                3  

解决方法

这有效。它缺少#books和#booksUpdate之间的BookId上的联接

select a.authorName,count(b.BookName) as countBooks,count(bu.BookName) as countBooksUpdate,(count(bu.BookName) + count(b.BookName)) as Total
from #Authors a   
     left join #books b on a.AuthorId=b.AuthorId  
     left join #booksUpdate bu on a.AuthorId=bu.AuthorId  
                                  and b.BookId=bu.BookId
group by a.authorName;

结果

authorName  countBooks  countBooksUpdate    Total
Ahmed       3           0                   3
Eslam       0           0                   0 
Mohamed     3           0                   3

#booksUpdate表中的BookId与#books表中的不匹配。没有BookId上的联接，查询将执行交叉联接。

假设#booksUpdate中的3行已更改，以匹配Ahmed的3行。在这里，我正在更改问题中发布的数据。

drop table if exists #booksUpdate;
go
create table #booksUpdate  
(  
BookId int,BookName nvarchar(200),AuthorId  int  
)  
insert into #booksUpdate  
values  
(122,'matlab',1),(233,'3dmax',(555,'c',1)  

然后结果如下：

authorName  countBooks  countBooksUpdate    Total
Ahmed       3           3                   6
Eslam       0           0                   0
Mohamed     3           0                   3