MySQL Left Outer Join not returning NULL values for COUNT(*)

Question

I am attempting to left outer join 2 tables: TABLE1 contains a list of users and TABLE2 contains a list of forms completed by users. I want to display the count of forms by user, created after a given date, where status equals COMPLETED.

The following query is working but not displaying NULL values (which I need):

select TABLE1.USERNAME, count(TABLE2.FORMS)
from TABLE1
left outer join TABLE2 on (TABLE2.USERID = TABLE1.USERID)
and TABLE2.STATUS = COMPLETED
where TABLE2.DATE > 20140801
group by TABLE1.USERNAME;

What do I need to do to include users with NULL counts ie no forms in TABLE2?

Thanks

Answer 1

You need to move the condition in the where clause to the on clause:

select TABLE1.USERNAME, count(TABLE2.FORMS)
from TABLE1 left outer join
     TABLE2
     on (TABLE2.USERID = TABLE1.USERID) and TABLE2.STATUS = COMPLETED and
        TABLE2.DATE > 20140801
group by TABLE1.USERNAME;

In rows that don't match, the table2 columns are set to NULL . That, in turn, causes your where clause to fail (because comparisons to NULL return NULL , which is treated as false).

When using left outer join , filters on the second table go in the on clause. Filters on the first table go in the where clause.