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, then the result for that record should be 0:

Data

My database looks like this (I'm not able to change the structure, unfortunately):

MESSAGE
----------------------------------------------
MESSAGEID   SENDER        SUBJECT
----------------------------------------------
1           Tim           Rabbit of Caerbannog
2           Bridgekeeper  Bridge of Death

MESSAGEPART
----------------------------------------------
MESSAGEID   PARTNO        CONTENT
----------------------------------------------
1           0             (BLOB)
1           1             (BLOB)
3           0             (BLOB)

(MESSAGEPART has a composite PRIMARY KEY("MESSAGEID", "PARTNO"))

Desired output

Given the data above I should get something like this:

MESSAGEID   COUNT(*)
-----------------------------------------------
1           2
2           0

It seems obvious that I need to do a left join on the MESSAGE table, but how do I return a count of 0 for rows where the joined columns from MESSAGEPART are NULL? I've tried the following:

Logic

I've tried

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

However, this returns

MESSAGEID   COUNT(*)
-----------------------------------------------
1           2
2           1

I've also tried

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

but this returns

MESSAGEID   COUNT(*)
-----------------------------------------------
1           2
            1

What am I doing wrong here?

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.