Counting number of joined rows in left join

Don't forget to use DISTINCT in case you will LEFT JOIN more than one table:

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

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.

You first want to count in your messaepart table before joining, i think. Try this:

   SELECT m.MessageId
        , COALESCE(c, 0) as myCount
     FROM MESSAGE m
LEFT JOIN (SELECT MESSAGEID
                , count(*) c 
             FROM MESSAGEPART 
            GROUP BY MESSAGEID) mp
       ON mp.MESSAGEID = m.MESSAGEID