Question

I want to run a query which grabs all courses. Each row (course) should also have a user_count.

Let me show my table structure real quick. I have two tables of relevance. This doesn’t show all the fields for these tables, just the relevant info.

COURSE
id
title
desc

USER_COURSE
id
user_id (fk)
course_id (fk)

My first stab at this was to create the following query:

SELECT course.title, course.desc, COUNT(*) as `user_count` 
FROM (`course`) 
JOIN `user_course` ON `user_course`.`course_id` = `course`.`id` 
GROUP BY `user_course`.`user_id`

The problem with this query is that it will only show courses which has records in the user_course table. Pointers?

Answer 1

Key points:

1. count(*) should be COUNT(user_course.user_id) as you are counting number of users per course
2. ON condition must be wrapped by parenthesis
3. Use group by course.id as you are grouping the result on per course basis.

See the query bellow.

    SELECT course.title, 
           course.desc, 
           COUNT(`user_course`.`user_id`) AS `user_count` 
    FROM   `course` 
           JOIN `user_course` 
             ON ( `user_course`.`course_id` = `course`.`id` ) 
    GROUP  BY `course`.`id`