Question

I have three tables: page, attachment, page-attachment

I have data like this:

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 

I would like to get the number of attachments per page also when that number is 0. I have tried with:

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

I am getting this output:

NAME        ATTACHMENTSNUMBER second page  2 third page   1 

I would like to get this output:

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

How do I get the 0 part?

Answer 1

Change your ‘inner join’ to a ‘left outer join’, which means ‘get me all the rows on the left of the join, even if there isn’t a matching row on the right.’

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