Counting number of joined rows in left join

Question

I\'m trying to write an aggregate query in SQL which returns the count of all records joined to a given record in a table; If no records were joined to the given record, the

Answer 1

How about something like this:

SELECT m.MESSAGEID, sum((case when mp.messageid is not null then 1 else 0 end)) FROM MESSAGE m
LEFT JOIN MESSAGEPART mp ON mp.MESSAGEID = m.MESSAGEID
GROUP BY m.MESSAGEID;

The COUNT() function will count every row, even if it has null. Using SUM() and CASE, you can count only non-null values.

EDIT: A simpler version taken from the top comment:

SELECT m.MESSAGEID, COUNT(mp.MESSAGEID) FROM MESSAGE m
LEFT JOIN MESSAGEPART mp ON mp.MESSAGEID = m.MESSAGEID
GROUP BY m.MESSAGEID;

Hope that helps.