MySQL - can't get a left outer join with a count(*) to return the items where count = 0

Question

I have this schema:

mysql> describe suggested_solution_comments;
+-----------------------+----------------+------+-----+---------+----------------+
| Field                 | Type           | Null | Key | Default | Extra          |
+-----------------------+----------------+------+-----+---------+----------------+
| comment_id            | int(10)        | NO   | PRI | NULL    | auto_increment |
| problem_id            | int(10)        | NO   |     | NULL    |                |
| suggested_solution_id | int(10)        | NO   |     | NULL    |                |
| commenter_id          | int(10)        | NO   |     | NULL    |                |
| comment               | varchar(10000) | YES  |     | NULL    |                |
| solution_part         | int(3)         | NO   |     | NULL    |                |
| date                  | date           | NO   |     | NULL    |                |
| guid                  | varchar(50)    | YES  | UNI | NULL    |                |
+-----------------------+----------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)

mysql> describe solution_sections;
+---------------------+---------------+------+-----+---------+----------------+
| Field               | Type          | Null | Key | Default | Extra          |
+---------------------+---------------+------+-----+---------+----------------+
| solution_section_id | int(10)       | NO   | PRI | NULL    | auto_increment |
| display_order       | int(10)       | NO   |     | NULL    |                |
| section_name        | varchar(1000) | YES  |     | NULL    |                |
+---------------------+---------------+------+-----+---------+----------------+

My query is this:

select   s.display_order, 
         s.section_name, 
         s.solution_section_id ,
         count(c.comment_id) AS comment_count    
FROM solution_sections s left outer join suggested_solution_comments c 
           ON (c.solution_part = s.solution_section_id) 
where      problem_id = 400    
group by   s.display_order, s.section_name, s.solution_section_id   
order by   display_order;

it returns only rows where there is a count > 0 but if the count is 0 it doesnt return those rows.

Any idea how to make it return all the rows? :)

Thanks!!

Answer 1

This is because the where problem_id = 400 removes rows with no corresponding suggested_solution_comments row. Moving the condition from the where filter to the on clause should address the problem:

select s.display_order, s.section_name, s.solution_section_id ,count(c.comment_id) 
AS comment_count
from solution_sections s     
left outer join suggested_solution_comments c 
ON (c.solution_part = s.solution_section_id) AND problem_id = 400
group by s.display_order, s.section_name, s.solution_section_id   
order by display_order;