SQL Query, Count with 0 count

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

Answer 2

Here's another solution using sub-querying.

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

Answer 3

Depending on the database, for speed, you could use the UNION command.

The SQL is longer, but, depending on the database, it speeds things up by seperating "count things that are there" and "count things that are not there".

(
select page.name, count(page-attachment.id) as attachmentsnumber 
from page 
inner join page-attachment on page.id=page-id 
group by page.id
)
UNION
(
select page.name, 0 as attachmentsnumber 
from page
where page.id not in (
    select page-id from page-attachment)
)   

The database I need this solution for has 20 pages in more than a million attachments. The UNION made it run in 13 seconds rather than so long I got bored and tried another way (somewhere above 60 seconds before I killed the outer join and subquery methods).

Answer 4

You want a left join, instead of an inner join, as that allows records to not exist.

Answer 5

LEFT join is your friend. To learn more about different join types refer to http://en.wikipedia.org/wiki/Join_(SQL)

Answer 6

Use this:

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