LEFT JOIN not creating NULL records for unmatched rows

Question

LEFT JOIN supposedly joins 2 tables and creates NULL values when no matching record is found in the second (right) table. In the following timesheet scenario, I can't seem to get that to happen.

users_table:

user_id
-------
frank
joe
jane

timesheet_table:

user_id    month    hours
---------------------------
frank          jan     1
frank          jan     2
frank          jan     3
frank          feb     5
frank          mar     10
joe            jan     8
joe            feb     5
joe            feb     10
jane           jan     5

mysql:

1 SELECT *
2 FROM users_table
3 LEFT JOIN(
4    SELECT
5        timesheet_table.user_id,
6        month,
7        SUM(hours)
8    FROM timesheet_table
9 ) timesheet_summary
10 ON users_table.user_id = timesheet_summary.user_id
11 GROUP BY month, users_table.user_id

Gives me something like this...

user_id    month    hours
---------------------------
frank        jan     6
joe          jan     8
jane         jan     5
frank        feb     5
joe          feb     15
frank        mar     10

But what I hoped for was...

user_id    month    hours
---------------------------
frank        jan     6
joe          jan     8
jane         jan     5
frank        feb     5
joe          feb     15
jane         feb     NULL
frank        mar     10
joe          mar     NULL
jane         mar     NULL

What is REALLY confusing me is that if I add a the following criteria between lines 8 and 9 it works (sort of)!

WHERE month = 'feb'

Gives me...

user_id    month    hours
---------------------------
frank        feb     5
joe          feb     15
jane         feb     NULL

Which is close but not exactly what I'm trying to do.

Answer 1

Actually that's what left join is supposed to do. You must use inner join instead.

See this for an overall idea