SQL查询,计数为0计数

Question

我有三个表:页面,附件,页面附件

我有这样的数据:

page
ID    NAME
1     first page
2     second page
3     third page
4     fourth page

attachment
ID    NAME
1     foo.word
2     test.xsl
3     mm.ppt

page-attachment
ID    PAGE-ID   ATTACHMENT-ID
1     2         1
2     2         2
3     3         3

我想在该数字为0时获得每页的附件数量.我尝试过:

select page.name, count(page-attachment.id) as attachmentsnumber 
from page 
    inner join page-attachment on page.id=page-id 
group by page.id

我得到这个输出:

NAME        ATTACHMENTSNUMBER
second page  2
third page   1

我想得到这个输出:

NAME        ATTACHMENTSNUMBER
first page   0
second page  2
third page   1
fourth page  0

如何获得0部分？

Answer 1

将"内部联接"更改为"左外部联接",这意味着"获取联接左侧的所有行,即使右侧没有匹配的行".

select page.name, count(page-attachment.id) as attachmentsnumber 
from page 
    left outer join page-attachment on page.id=page-id 
group by page.name

Answer 2

这是使用子查询的另一种解决方案.

SELECT
  p.name,
  (
    SELECT COUNT(*) FROM [page-attachment] pa
    WHERE pa.[PAGE-ID] = p.id
  ) as attachmentsnumber
FROM page p