Getting the recordcount of a third table from a Join query

Question

Hi Folks,
I've been trying to get my head around the best way of doing
this within a single query, and sadly none of the books within my
collection touch upon this scenario.
I'm using the query below to query 2 tables as you can see. I
want to add in a further query which searches a third table
'ccsample' looking for matching records in the 'merchant_id' field
against the merchant_id field of the cc_watchlists table. I only
want to return the recordcount from the 'ccsample' table.
I've tried adding another join, however if the recordcount in
ccsample is 0, no records are returned at all (the query below
returns 2 rows).
I hope this makes sense!
Query;
<cfquery name="getwatched" datasource="#datasource#">
SELECT r.retailer_name, r.id, r.shortname, d.id AS wid,
d.merchant_id, d.user_id, d.e_updates
FROM cc_watchlists d
LEFT JOIN merchants r
ON d.merchant_id=r.id
WHERE d.user_id=#SESSION.Auth.ID#
ORDER BY r.retailer_name ASC
</cfquery>

Answer 1

Did you make it a LEFT (OUTER) JOIN as well (something like
below)? Not sure about what database you are using, but the
ISNULL() function in MSSQL will convert any instances where you
don't have a match to a 0 for counting purposes.
SELECT r.retailer_name, r.id, r.shortname,
d.id AS wid,d.merchant_id, d.user_id, d.e_updates,
COUNT(ISNULL(c.merchant_id, 0)) AS cc_count
FROM cc_watchlists d
LEFT JOIN merchants r ON d.merchant_id=r.id
LEFT JOIN ccsample c ON d.merchant_id=c.merchant_id
WHERE d.user_id=#SESSION.Auth.ID#
GROUP BY r.retailer_name, r.id, r.shortname,
d.id,d.merchant_id, d.user_id, d.e_updates
ORDER BY r.retailer_name ASC
Phil