Joining two tables using a count
I have two tables as defined below:
EMAILS
ID SUBJECT BODY
1 Test1 blah blah
2 Test2 blah blah blah
3 Test3 more blah
ATTACHMENTS
ID EMAIL_ID ATTACHMENT NAME
1 2 TEST.DOC
2 2 TEST.XLS
I'm trying to perform a select to get back all of the emails and a count of how many attachments (if any) for each row in the EMAILS table -- preferably in a single query where the results would resemble something like:
EMAIL_ID SUBJECT NUM_ATTACHMENTS BODY
1 Test1 0 blah blah
2 Test2 开发者_如何学Go 2 blah blah blah
3 Test3 0 more blah
I've tried everything but the "right way" and if anyone has a few minutes to help me create the right SQL to build this, I'd greatly appreciate it. Thanks.
I like brydgesk's solution, but would prefer not to use a subquery when possible.
This query will display 0 when there are no attachments for a given email.
SELECT
a.email_id,
e.subject,
e.body,
COALESCE(count(a.id),0) AS NUM_ATTACHMENTS
FROM
emails e
LEFT OUTER JOIN attachments a
ON e.id = a.email_id
GROUP BY
a.email_id, e.subject, e.body
Have you tried something like this? (untested)
SELECT
e.id,
e.subject,
count(*),
e.body
FROM
(SELECT *
FROM emails e
JOIN attachments a
ON e.id = a.email_id)
GROUP BY e.id, e.subject, e.body
You can do this as a simple subquery in the SELECT clause:
Select E.ID As EMAIL_ID
, E.SUBJECT
, ( Select Count(*)
From ATTACHMENTS As A
Where A.EMAIL_ID = E.ID ) As NUM_ATTACHMENTS
, E.BODY
From EMAILS As E
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