如何解决为作者相关的书算错了为什么以及如何解决?
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
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