SQL COUNT() function and LEFT OUTER JOIN

Question

I have two tables, users and departments. I want to have table, where are two columns: first is department name, second is count - how many users are assigned to this department. And I have this piece of code:

SELECT department_name as 'deptName', 
       COUNT(users.department_id) as 'userCount' 
FROM departments
LEFT OUTER JOIN users
ON departments.id = users.department_id 
GROUP BY 'deptName'

Department's table columns are:

 integer id PK

 varchar(20) department_name

User's table columns are:

 integer id PK

 varchar(20) name

 varchar(20) surname

 int department_id FK

But it does not work.

Now I have 2 departments, and output should be 2 rows, first with count 8 and second with count 1. But I see only one row, with all count (9). I use MySQL installed with XAMPP.

Answer 1

SELECT department_name as 'deptName',
       COUNT(users.department_id) as 'userCount'
  FROM departments
  LEFT OUTER JOIN users
    ON departments.id = users.department_id
 GROUP BY `deptName`

Notice the tick marks vs. your single quotes in the GROUP BY (this is the key to the left of the 1 on your keyboard). Refer to: http://dev.mysql.com/doc/refman/5.0/en/problems-with-alias.html

You could also just group by department_name (the field itself, rather than the alias)

Currently you are grouping on the literal value 'deptName', not the field that you've given an alias of deptName, which is why you only have 1 row returned. You're not actually doing any grouping.